304 Prof. A. W. Rucker on the Density and 



earth; and G (i. e> N) through a high-resistance galvanometer 

 to earth, the indications of the galvanometer being therefore 

 due only to current discharged from N. A much larger 

 instrument constructed on the same lines was used for needle 

 A and other measurements. The body of this instrument is 

 a tin cylinder 43 centim. long (horizontally) and 23 centim. 

 in diameter. The needle is suspended in a small metal box 

 at the centre, and the electrified plate is supported like C D 

 (fig. 1) from one end. 



The tilting is effected in each of the above instruments by 

 fixing them to a brass base, provided with pivots at one end 

 and a vertical micrometer-screw with large divided head at 

 the other. This method works admirably. 



In conclusion I wish to express my thanks to my friend 

 Mr. F. B. Fawcett, a former student of University College, 

 Bristol, and to my assistant Mr. J. Quick, for much careful 

 help in carrying out the measurements described in this paper. 

 To Prof. Lodge my thanks are so numerous that I cannot 

 express them. His kindness, both by word and by deed, has 

 been unceasing. Indeed but for him this paper would pro- 

 bably never have been written. 



University College, Bristol. 



XXXVII. On the Relation betiveen the Density and Composition 

 of Dilute Sulphuric Acid. By A. W. Rucker, F.R.S.* 



SOME time ago I had occasion to examine closely Mr. 

 Pickering's method of detecting breaks in curves re- 

 presenting the results of experiments, and in consequence of 

 the recent controversy on the subject I have lately reconsidered 

 the matter. 



Mr. Pickering's work covers a wide area, and he cannot 

 complain if those who study his method devote themselves 

 particularly to some one application of it. 



I have specially investigated the curve which expresses 

 between 46 per cent, and 80 per cent, the relation at 18° 

 between the percentage strength of the solution and the 

 " first differential 9> of the densities. 



This was chosen on two grounds. First, because it in- 

 cludes within its range the percentage corresponding to the 

 hydrate which Mr. Pickering has isolated. Secondly, because, 

 although Mr. Pickering estimates the magnitude of the 

 possible error of observation to be greater at 18° than at 

 other temperatures, the agreement between his curves and 



* Communicated by the Author. 



