1872.] Mr. W. Crookes on the Atomic Weight of Thallium. 477 



highly rarefied atmosphere, — it cannot be said in vacuo, owing to the diffi- 

 culty of working under such a difference of pressure between the attno - 

 sphere of the balance and that surrounding it. 



The Balances. 



Two balances were used. That which I shall call the a*>-balance was 

 made by Messrs. Keissler and Neu expressly for this work, and will clearly 

 indicate a difference of O'OOOl of a grain when loaded with 1000 grains in 

 each pan. 



The second balance, which I shall call the vacuu?n-h&\mce, is almost a 

 duplicate of the first, of 14-inch beam, with agate knife-edges and planes, 

 made by (Ertling. It is enclosed in a cast-iron case connected with an air- 

 pump, and so arranged that I can readily weigh any substance in air of any 

 desired density, the rarefaction being measured by a barometer-gauge. 



At first it was attempted to put nearly the correct weight into the pan, 

 and then make the final adjustment by means of the rider. It was, how- 

 ever, soon found that a nearer approach to accuracy is to introduce a certain 

 weight, and then to alter the pressure of the air until the balance shows 

 equilibrium. Two weighings at different degrees of atmospheric pressure, 

 varying by a considerable interval, give data upon which to calculate what 

 the weight would be in a perfect vacuum. 



The Weights. 



A set of weights as ordinarily supplied by even the best instrument- 

 makers is never absolutely exact ; however carefully they may be adjusted, 

 the pieces of metal which respectively represent 1000 grains, 100 grains, 

 10 grains, &c. are only more or less approximations to the true weights. 

 In most chemical analyses, the error arising from such inaccuracies in the 

 weights used is so small in comparison to errors of manipulation, or to im- 

 perfections inherent in the chemical processes adopted, that it may gene- 

 rally be disregarded ; but when the chemist has for his object the deter- 

 mination of an atomic weight, or is engaged in other researches demanding 

 the highest refinement of accuracy which chemistry and physics can supply, 

 then he is bound to neglect no correction which will increase the precision 

 of the results. 



The weights I employed were of platinum, made expressly for these in- 

 vestigations by Messrs. Johnson and Matthey. The platinum was quite 

 pure ; it was fused, cast, and then well hammered. The weights were 

 adjusted by myself during May, June, July, and August 1864: they 

 were first roughly adjusted, and then the specific gravity of each weight 

 was taken. The weights were heated to redness in a bath of magnesia pre- 

 vious to ascertaining their specific gravity. The density of the larger 

 weights was ascertained to the second place of decimals, and that of the 

 smaller ones to the first place. The following are the results of the final 

 adjustment, the weight in vacuo being calculated by the formula : — « 



