92 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



the products of the decomposition may he collected with such accuracy, as to afford a 

 very excellent and valuable measurer of the electricity concerned in their evolution. 



733. The forms of instrument which I have given, figg. 9, 10, 11. (709. 710. 711.), 

 are probably those which will be found most useful, as they indicate the quantity of 

 electricity by the largest volume of gases, and cause the least obstruction to the pass- 

 age of the current. The fluid which my present experience leads me to prefer, is a 

 solution of sulphuric acid of specific gravity about 1*336, or from that to specific 

 gravity 1*25 ; but it is very essential that there should be no organic substance, nor 

 any vegetable acid, nor other body, which, by being liable to the action of the oxygen 

 or hydrogen evolved at the electrodes (773. &c.), shall diminish their quantity, or add 

 other gases to them. 



734. In many cases when the instrument is used as a comparative standard, or even 

 as a measurer, it may be desirable to collect the hydrogen only, as being less liable 

 to absorption or disappearance in other ways than the oxygen ; whilst at the same 

 time its volume is so large, as to render it a good and sensible indicator. In such 

 cases the first and second form of apparatus have been used, figg. 7, 8. (707. 7O8.). 

 The indications obtained were very constant, the variations being much smaller than 

 in those forms of apparatus collecting both gases ; and they can also be procured 

 when solutions are used in comparative experiments, which, yielding no oxygen or 

 only secondary results of its action, can give no indications if the educts at both elec- 

 trodes be collected. Such is the case when solutions of ammonia, muriatic acid, 

 chlorides, iodides, acetates, or other vegetable salts, &c., are employed. 



735. In a few cases, as where solutions of metallic salts liable to reduction at the 

 negative electrode are acted upon, the oxygen may be advantageously used as the 

 measuring substance. This is the case, for instance, with sulphate of copper. 



736. There are therefore two general forms of the instrument which I submit as a 

 measurer of electricity. One, in which both the gases of the water decomposed are 

 collected (709.710.711.) ; and the other, in which a single gas, as the hydrogen only, 

 is used (707. 7O8.). When referred to as a comparative instrument, (a use I shall now 

 make of it very extensively,) it will not often require particular precaution in the ob- 

 servation ; but when used as an absolute measurer, it will be needful that the baro- 

 metric pressure and the temperature be taken into account, and that the graduation 

 of the instruments should be to one scale ; the hundredths and smaller divisions of a 

 cubical inch are quite fit for this purpose, and the hundredth may be very conveni- 

 ently taken as indicating a degree of electricity. 



737. It can scarcely be needful to point out further than has been done how this 

 instrument is to be used. It is to be introduced into the course of the electric current, 

 the action of which is to be exerted anywhere else, and if 60° or 70° of electricity are 

 to be measured out, either in one or several portions, the current, whether strong or 

 weak, is to be continued until the gas in the tube occupies that number of divisions 

 or hundredths of a cubical inch. Or if a quantity competent to produce a certain 



