1 62 
Dr. Thomson on some of 
choose to repeat this experiment ; but it had been made with 
such scrupulous attention, that I am satisfied it must approach 
very near the truth. 
As 3’966 differs by less than one per cent from 4, we can 
scarcely hesitate about adopting 4 as the true atomic weight 
of metallic chromium. Indeed 4 must be the true number, if 
we adopt the law of Dr. Prout, which I have so amply con- 
firmed by numerous examples in my “ First Principles of 
Chemistry/’ namely, that the atomic weights of all bodies 
are multiples of 0*125, the atom of hydrogen: for 0*125 x 
32 = 4. 
This conclusion will be corroborated by the experiments 
immediately to be detailed, which leave no doubt that the 
atomic weight of green oxide of chromium is 5. Now, as this 
oxide is a compound of 1 atom oxygen + 1 atom chromium, 
the atom of chromium must necessarily be 4. If we allow an 
error in weighing to have taken place in the preceding ex- 
periment to the amount 0*02 grain, the number 4 would come 
out as the result of the experiment. Now, I need hardly 
remark to practical chemists, how very difficult it would be 
to guarantee any similar experiment from so small an error 
as -^th of a grain. 
II. Green oxide of chromium. 
This oxide is always obtained by depriving chromic acid of 
a portion of its oxygen. Many processes have been followed. 
When chromate of potash is digested with alcohol or tartaric 
acid, the chromic acid is pretty speedily converted into green 
oxide. A solution of chromate of potash speedily lets green 
oxide fall when a current of sulphurous acid is passed through 
