1'314.] On the Cause sf Chemical Proportions, 101 



proportions prescribed by Bucholz for the preparation of molybdous 

 acid, that this acid is proportional to the sulphuret of molybdenum, 

 that is to say, Mo + 2 0, while the raolybdic acid is composed of 

 Mo + 3 O. 



But to assure myself of the accuracy of this determination, I 

 thought it requisite to examine the capacity of saturation of ni©- 

 lybdic acid. Having made some vain attempts to analyse molybdate 

 of lead and molybdate of barytes, I found that the only nietliod of 

 obtaining an exact result was to form molybdate of lead. I dis- 

 solved ]0 parts of neutral nitrate of lead in water, and poured an 

 excess of molybdate of ammonia into the liquid (the molybdate was 

 crystallized in a mother water strongly alkaline ; of course it was 

 neutral. This deserves notice, because there is a supermolybdate 

 of ammonia which is always formed when we attempt to concen- 

 trate a solution of neutral molybdate by evaporation). The molyb- 

 date of lead washed, dried, and heated to redness, weighed 1 1 068. 

 The liquid from which it had been precipitated did not, when 

 mixed with sulphate of ammonia, exhibit any traces of lead; there- 

 fore these lI-0f.J8 of molybdate of lead contain 67*3 per cent, of 

 oxide of lead. Hence the salt is composed of 



Molybdic acid 39-194 100 



O.Kideof lead 60-806 155-15 



100-000 



Tlie 155*15 of oxide of lead contain 11-093 of oxygen. Now 

 11 -093 X 3 = 33-27^. Therefore molybdic acid is composed of 



Molybdenum 66-721 ... .100 



Oxygen 33-279 49-88 



100-000 



This result is nearly a mean of the experiments of Bucholz. 

 Hence the volume of molybdenum must weigh 001-56; and its 

 fuboxide should be Mo + O. I must not conceal, however, that 

 its analogy with arsenic and chromic acids renders it probable that 

 molybdic acid contains 6 volumes of oxygen. 



3. Chromium {C\\). — Nobody has hitherto made exact experi- 

 ments on tiie quantity of oxygen which this metal absorbs in its 

 dita-rent degrees of oxidation. M. Vauquelin established only that 

 throinic acid appears to contain about 40 per cent, of oxygen. 



1 made the following experiments to determine this point. I 

 prepared chromate of lead and chromate of barytes by precipitating 

 a solution of neutral ciiromate of potash by nitrate of lead and by 

 muriate of barytes. 



A. Chromate of Lead. — A solution of 10 parts of nitrate of lead 

 prccij)itated by chromate of potash yielded 9-877^ parts chromate 

 *)i lead. The residual liquid gave no traces of lead when treated 



