420 MM. Svanberg and Struve on the Atomic Weight 



precipitate, which is very bulky in the moist state, shrinks 

 considerably in drying. This salt is likewise always obtained 

 when an excess of nitric acid is added to a solution of mo- 

 lybdic acid in carbonate of potash, and the whole set aside 

 for some time. Besides the trimolybdate of potash some other 

 molybdates separate, which are easily recognized by their cry- 

 stalline form, and also some salts which are not crystalline, 

 and which we shall subsequently describe. As soon as the 

 separation has taken place, the whole is brought upon a filter 

 and washed well with cold water; this dissolves the other cry- 

 stalline salts, a portion of which is even converted into trimo- 

 lybdate. The precipitate is then treated with boiling water, 

 which dissolves the trimolybdate, and the solution filters. 

 The salt subsequently separates gradually from the solution. 



"When this salt has separated quickly it forms a bulky 

 white precipitate, which, under the compound microscope, 

 is seen to be made up of innumerable white needles. When 

 however the separation has been very slow, it is acicular 

 and with a beautiful silky lustre, especially when stirred 

 under water with a glass rod ; the salt retains this lustre when 

 dry. When once separated it is almost insoluble in cold 

 water, but is dissolved in large quantity by boiling water, and 

 from this solution it separates very slowly, sometimes re- 

 quiring several weeks. In liquids containing other salts, as 

 for instance nitre, it is almost insoluble even when boiling. It 

 contains water of crystallization, which it parts with on calci- 

 nation, then melts, and solidifies on cooling into a crystalline 

 mass. It is then very sparingly soluble even in boiling water. 



Analysis. — I. 0*6282 grm. of the fused salt obtained by de- 

 composition of the double salt and dried in the air gave 0*219 

 sulphate of potash = 0*1189 potash. 



II. 1*0479 grm. salt of the same preparation lost on calci- 

 nation 0*1009 water. 



III. 0*7255 grm. of the same preparation lost on calcina- 

 tion 0*0709 water. 



IV. 2*6185 grms. fused salt gave 0*8810 sulphate of potash, 

 or 0*4761 potash. 



V. 1*408 grm. salt precipitated by nitric acid and dried in 

 the air lost at a red heat 0*632 water, and gave 0*4395 sul- 

 phate of potash = 0*2375 potash. 



The per-centage composition of the air-dried salt is accord- 

 ingly — 



II. III. V. Mean. 



Potash ... 16*869 16*869 



Molybdicacid ... ... 73*756 73*756 



Water . . . 9*628 9*773 9*375 9*592 



