We should carefully avoid mixing with white lead substances which may impair its brightness, since a pure white is its main quality. We shall see further on what its composition is. ,,White lead should be kept in closed vessels, otherwise it will acquire a brown shade. It forms the basis of a great many pigments. It should, for good paintings, be pure and without admixtures ; however, house painters add to it variable proportions of chalk or Meudon white, but the painting is without consistency or durability. Here is the process indicated by Watin for distinguishing white lead from chalk. ,A hole is made in a piece of charcoal, which is then ignited and a pinch of white lead thrown in. Air is blown upon the charcoal in order to keep up the combustion, and the white lead first turns yellow, and after a few minutes becomes reduced to bright globules of metallic lead. Chalk (carbonate of lime), on the other hand, may lose its carbonic acid by the operation, but the lime will remain as a white powder upon the charcoal. ,,If we desire to ascertain the proportion of carbonate of lime mixed with the carbonate of lead, we weigh 100 grammes of the sample, and mix them with 50 grammes of charcoal powder. The whole being smelted in a crucible, a button of metallic lead is produced, which is weighed. We then add 24 per cent, to the number obtained, and subtracting this sum from the previous 100 grammes, the difference is the weight of the carbonate of lime. The 24 per cent, repvesent about the weight of the carbonic acid, water, and oxygen separated from the carbonate of lead. ,,These two tests would be sufficient if carbonate of lime were the only foreign substance of the mixture. But as sulphate of lead may also be present, we are obliged to employ tests by the wet way. For instance we put 25 grammes of the sample into a glass flask, and pour gradually upon it nitric acid diluted with six times its weight of water. The acid is added as long as an effervescence takes place, and we heat the vessel gently. If all the substance be dissolved, we conclude that there is no sulphate of lead. Should there be an insoluble deposit, we throw the whole upon a filter and wash it thoroughly. The sulphate of lead collected is then dried and weighed. Let us suppose that its weight is 7 grammes. The filtered liquor contains chalk and the dissolved white lead ; their separation is effected by adding ammonia until the liquor smells of it slightly. The precipitate of oxide of lead is collected upon a filter, washed,dried and weighed. Let the supposed weight be 10 grammes ; we know that 100 parts of oxide of lead are equal to 1 19.78 parts of white lead ; therefore 10 grammes of oxide represent 11.978 grammes of white lead, that is, 12 grammes without fractions. Deducting from the 25 grammes of sample the sum of the weights of sulphate and carbonate of lead, there remain 6 grammes of carbonate of lime. Lime may be ascertained and determined by pouring into the liquor, filtered from the oxide of lead, a solution of oxalate of ammonia which will produce a white precipitate of oxalate of lime. The results are only approximate, as we do not wish to complicate the operations which require a certain amount of chemical knowledge in order to operate with certainty. We have also found samples of white lead which were mixed with sulphate of baryta or clay; these sub- ,stances remain in the insoluble residuum. ,Here are the processes employed by Mr. Louyet for ascertaining the impurities of white lead : ,"I was intrusted, some time since, with three different samples of white lead, intended for exportation. It is probable that the destination of these products induced the manufacturer to think that it was useless to remain within bounds, and that the ignorance of the consumers would prevent them from ascertaining that what was sold as white lead could be as properly called sulphate of baryta as white lead. ,,"One gramme of sample No. 1, heated to redness in a platinum crucible until complete transformation into oxide of lead, gave a loss of . 0.100 gramme. ,,"A second calculation gave the same weight. ,," For one gramme of sample No. 2, calcined in the same manner, the loss was ... 0.049 gramme. ," Sample No. 3, 1 gramme, loss . . . .... 0.037 ".,," The calcined product No. 1 was boiled with pure nitric acid, then water was added and the boiling continued. The insoluble residuum was yellowish, although the liquor was strongly acid. After filtration the residue was well washed with boiling water and calcined. Its weight was 0.305 gramme after deducting the ashes of the filter. ,,"I will observe that the residuum of No. 1 was darker than that of No. 2, and this latter darker than No. 3, which was quite white. The residuum of No. 1, after being heated with the blowpipe upon charcoal and with soda, stained a permanent black the piece of silver upon which it had been put wet. ,,"This is a characteristic of sulphates. It was proved that the sulphate mixed with the carbonate of lead was sulphate of baryta, by boiling it with a solution of carbonate of soda, filtering, and dissolving the washed residue upon the filter with hydrochloric acid. The liquor obtained gave a heavy white precipitate with sulphuric acid. ,,"The solution resulting from the treatment of white lead 2sTo. 1 with nitric acid was precipitated with sulphuric acid, and the calcined sulphate of lead weighed 0.765 gramme, corresponding to 0.563 gramme of oxide of lead, or 0.674 gramme of neutral carbonate. Calculated from the proportion of oxide of lead, that of carbonic acid is 0.111 gramme, whereas the loss by calcination of the white lead is only 0.100 gramme. I admit that this difference is due to the fact that all of the oxide of lead is not carbonated, but that a certain proportion remains in the hydrated state. But, as the equivalent of water is smaller than that of carbonic acid, it follows that the number ,is too high if we suppose that all of the oxide is combined with carbonic acid, and we must subtract 0.011 ,from 0.674 ; there remains 0.663 gramme. White lead ~No. 2 was treated in the same manner, and ,the residue insoluble in nitric acid weighed 0.660 gramme after being washed and calcined. The proportion of sulphate of lead from the nitric solution was 0.360 gramme, corresponding to 0.264 gramme of protoxide of lead. But here the number calculated for carbonic acid differs but little from that found by direct experiment. In this case, as in the former, the number obtained for carbonate of lead is a little low, and the loss may be added to it. Indeed, the sulphate of lead is slightly soluble in acid liquors, and the precipitation by the oxalate of ammonia ,would have given more accurate results. It follows that the calculated number for carbonic acid would have been a little higher, and above that found by direct experiment. ,," But I repeat the observation already made, that a portion of the oxide of lead in white lead is in the hydrated state. One gramme of the sample No. 3 gave an insoluble residuum equal to 0.718 gramme, and a precipitate of sulphate of lead weighing 0.277 gramme, which corresponds to 0.203 of oxide of lead or 0.243 of carbonate of lead. ,," The composition of the samples was, therefore, as follows: ,, White lead. Sulphate of baryta. ,,1 gramme sample No. 1 . . . 0.695 0.305 ,,1 " . " No. 2 ... . 0.340 0.660 ,,1 " " No. 3 . . . 0.282 0.118 ,," These analyses, the last one especially, show that I was right in saying that these products may indifferently be called white lead or sulphate of baryta." ,,"White lead is often mixed with that sulphate of baryta which is called blanc fixe or baryta white, andwhich is prepared from the carbonate of baryta. It is an adulteration which ceases to be objectionable when the manufacturer makes the composition known. ,Belgian and German manufacturers sell various qualities of white lead, the compositions of which are ,known by the names they bear; thus : ,1. Krems white is a pure white lead ; ,2. Venice white is a mixture of equal parts of siphate of baryta and white lead ; ,3. Hamburg white is a mixture of ,Sulphate of baryta . . . . .2 parts. ,White lead 1 part. ,4. Holland white is composed of ,Sulphate of baryta 3 parts. ,White lead ... ... 1 part. ,,A bluish tinge is often imparted to white lead with a small proportion of indigo. ,,It is said that the following mixtures are generally found in the French color trade : ,, White lead. Sulphate of baryta. ,,White lead (superfine) ... 85 15 ,No. 1 . . . 70 30 ,No. 2 . . . 60 40 ,No. 3 . . . 40 to 50 60 to 50 ,,In order to ascertain whether a sample of white lead is mixed with sulphate of baryta or sulphate of lead, it is treated with nitric acid diluted with two or three parts of distilled water. Pure white lead is entirely dissolved, whereas the above sulphates remain unacted upon by the reagent. ,,Mr. A. Bacco has indicated a simple process by which white leads may be tested by the wet way. ,,"The quality of white leads," says he, "as every chemist knows, depends on their extreme opacity, which is the greater as their molecules are amorphous and their composition more basic. In experimenting upon a crystalline white lead, with little body, I have ascertained that it may be improved by digesting it in a carbonated alkaline solution, rendered slightly caustic by an addition of a small quantity of quicklime. The entire operation is performed in the cold. ,,"But as a white lead should be basic, in order to have body, I have tried a process for ascertaining whether a white lead is more or less basic, and I have succeeded with a solution of neutral chromate of potassa poured upon wet white lead. This latter substance is converted into a chromate of lead, which is neutral, basic, or six basic, according as the white lead itself is more or less basic, and the degree is shown by a change of coloration. ,,"If the chromate be lemon-yellow, the white lead is neutral and of inferior quality; an orange color indicates a white lead slightly basic, but if the color be a scarlet-red we may be sure that the white lead is highly basic. Indeed, every chemist knows that the neutral chromate of lead is lemon-yellow, the tribasic orange-yellow, and the six-basic fire-red. ,,"We may then conclude that the degree of coloration thus obtained gives a sure indication of the quality of white leads." ,,All these methods of ascertaining the purity of white leads appear to us far from entirely satisfactory ; and, by reading the following paragraph, it will be seen that the composition of this product is not so simple as is supposed, and that a thorough chemical analysis will alone give correct indications of the quality of white lead.