402 II S. Garhart— Standard Clark Cell 



the assigned values of the masses of the planets Mars, Venus 

 and Mercury, based presumably on more accurate data or im- 

 proved methods of computation, have been in the direction of 

 the values which the law in question would from their observed 

 diameters have assigned to them. For the full confirmation 

 of the law it may yet be considered necessary by the cautious 

 investigator to have some additional proof of its correctness. 

 In what direction it is best to seek for additional proof of a 

 decisive character it is difficult to say. Could the law of in- 

 crease of density within the earth which would result from the 

 derived law of the planets' densities, be deduced, evidence of 

 the correctness of the law might be found in the same way 

 that Thomson and Tait have in their treatise on Natural Phil- 

 osophy, Part II, section 828, sought for evidence of the cor- 

 rectness of Laplace's hypothesis. It is to be hoped that more 

 accurate determinations may be made of the masses and diam- 

 eters of the planets Venus and Mercury, and of the satellites 

 of Jupiter. It is in this direction that the writer has most 

 hope for the full confirmation of the law in question. 



Art. L. — An improved Standard Clark Cell with low 

 Temperatiwe- Coefficient ; by H. S. Cakbart. 



The best form of Clark cell hitherto made is that of Lord 

 Rayleigh, described in the Philosophical Transactions for 1885. 

 The objections to this form are that the temperature-coefficient 

 is not the same for all cells, as is shown in Lord Rayleigh's 

 paper, and it is so high as to introduce a very troublesome and 

 uncertain error because of the difficulty of ascertaining the 

 exact temperature of the cell ; second, it is not so constructed 

 mechanically as to prevent the mercury from coming into con- 

 tact with the zinc when the cell is subjected to violent jars in 

 transportation ; thirdly, a great chemical defect is the facility 

 with which local action takes place between the zinc and the 

 mercury salt. I might add that the mercurous sulphate, pur- 

 chased by Lord Rayleigh, evidently contained considerable salt 

 in the mercuric form, as is shown by its turning yellow on 

 mixing with the zinc sulphate solution. 



All these difficulties I have, at least in large measure, over- 

 come. Respecting the materials, the greatest care is required 

 to secure and maintain cleanliness and purity in their prepara- 

 tion. The mercury must be distilled in vacuo after being 

 cleaned by chemical means. The zinc sulphate should be free 

 from iron as well as other impurities. The mercurous sulphate 

 can be made almost or quite free from the mercuric form by 



