Cavendish's Chemical Equivalent Weights 27 



than that of rain water, and that of rain water was 2-4 times less than that 

 of distilled water*. 



In January 1777, he found that salt in 2999 conducted about 70 or 

 90 times better than some water distilled in the preceding summer but 

 only about 25 or 50 times better than the distilled water used in the year 

 1776 f , and that the conductivity of distilled water increased by standing 

 two or three hours in a glass tubej. 



He also found that in order to make the conducting powers of his 

 weakest solutions of salt agree with the hypothesis that they are as the 

 quantity of salt in them, it would be necessary only to suppose that his 

 distilled water contained one part of salt in 120,000 . 



It was found that distilled water impregnated with fixed air from oil 

 of vitriol and marble conducted 2\ times better than the same water 

 deprived of its air by boiling ||, and that the presence of absorbed air in 

 a weak solution of salt seemed to increase its conductivity \ 



In order to find whether electricity is resisted in passing out of one 

 medium into another in perfect contact with it, Cavendish prepared a 

 tube containing 8 columns of saturated solution of sea salt enclosed be- 

 tween columns of mercury. He found that the shock was diminished in 

 passing through a mixed column in which the length of salt water was 

 21-8 inches as much as in passing through a single column of the same 

 size whose length was 22-94 inches**. 



The difference would have been far greater if the comparison had been 

 made with an ordinary galvanometer and continued currents which rapidly 

 produce polarization, but with the small quantities of electricity which 

 Cavendish used, the effect of polarization would hardly be sensible. 



He also made a compound conductor consisting of 40 bits of tin 

 soldered together. The shock through this appeared to be of the same 

 strength as through a single piece of the same size. This experiment 

 however is not of much value, as the resistance of the conductor was far 

 too small compared with that of Cavendish's body to give good results ff. 



We now come to a very remarkable set of experiments which Cavendish 

 made on a series of salts and acids in order to determine their relative 

 electric resistance. They are recorded in Arts. 626, 627 and 694, and are 

 dated Jan. 13 and 15, 1777. 



The strength of the different solutions was such, as Cavendish tells us, 

 "that the quantity of acid in each should be equivalent to that in a 

 solution of salt in 29 of water." 



* Art. 525. f Art. 690. J Art. 621. 



Art. 630. || Arts. 625, 693. f Art. 692. ** Art. 578. 



ft Art. 579. The resistance of a man's body, from one hand to the other, varies 

 from about 1000 ohms when the hands are well wetted with salt water, to about 

 12,000 when the hands are dry. When the outer skin is removed by a blister, the 

 resistance is very much diminished. The resistance of the compound conductor was 

 probably a fraction of an ohm. See Note 31. 





