8 On the Probability of Error January, 



which I term a vacuum balance. It was made by Oertling, 

 and is a duplicate of the first, but it is enclosed in a cast- 

 iron case, connected with an air-pump, and arranged for the 

 weighing to take place in air of an)' desired density. The 

 best manner in which to use such a balance as this is to in- 

 troduce a certain approximate 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. 

 For the full elucidation of the formulae employed, for the 

 method of adjusting the standard grain-weights according 

 to their value in vacuo, and for the preparation of the glass 

 apparatus and the pure reagents, I must refer the reader to 

 my paper in exicnso, contributed to the Royal Society.* 

 But I may, from the abstract of that paper, collect the 

 results of a series of the weighings. They were as follows: — 



The weight of the glass + thallium. 



The weight of the glass -f- nitrate of thallium. 



The weight of the glass alone. 



Grs. 



True weight of thallium in vacuo . =183*790232 

 True weight of nitrate of thallium 



actio =239*646066 



True weight of glass =766-133531 



(a) Weight of thallium according 



to true value of weights in air =153*783921 



(b) Weight of nitrate of thallium in 



air (1005-425937—765-814578) =239*611359 



(c) Weight of glass, <5cc, in air . =765*814578 

 Weights employed to balance (a) ► =183*5099 

 Weights employed to balance {b) 



(1005-4364-765-8081) . . =239*6253 

 Weights employed to balance (c) . =765*8081 



From these data the atomic weight can be deduced by 

 simple proportion, but the results of the statements of the 

 proportion are absolute only if the atomic weights of nitro- 

 gen and oxygen are correct. The determinations of Prof. 

 Stas show that the atomic weights of nitrogen and oxygen 

 should be represented by N = 14*009, and 3 = 47*880, in- 

 stead of X = 14 and O = 16, as hitherto more generally held. 

 The equivalent of nitric acid thus becomes N0 3 = 6i"88g, 

 instead of the old equivalent X0 5 = 62. Taking as data 

 Prof. Stas's determination of the atomic weights of nitrogen 



* June, : 



