186 Proceedings of Royal Society of Edinburgh. [sess. 
simply dissolved in the palladium, forming a solid solution. The 
formation of a definite chemical compound, on the other hand, 
would agree better with the facts, for the electromotive force 
would then be practically independent of the quantity of com- 
pound formed. The electromotive force of a metal dipped in an 
electrolyte, for example, is practically independent of the quantity 
of metal employed. 
Whilst these electro-chemical experiments would seem to indi- 
cate the formation of a definite chemical compound, it still remains 
uncertain whether the compound so formed is Pd 3 H 2 , or whether it 
contains relatively more hydrogen than is represented by this 
formula. Experiments at low temperatures, such as those already 
suggested, might throw some light on this question. 
Quite recently a new criterion for determining whether definite 
chemical compounds are contained in alloys, and if so what their 
composition is, has been brought forward by Liebenow (Zeits. f. 
Elektrocliemie , iv. 201). 
This method, which is based on measurements of the electrical 
resistance of the alloys or on their temperature coefficients, is 
unlikely to be of any assistance in the case of palladium hydrogen, 
for, in order to obtain satisfactory results, it would be necessary to 
prepare a large number of alloys, ranging from pure palladium, on 
the one hand, to pure hydrogen on the other ; whereas, so far as we 
Enow at present, it is impossible to introduce as much as one per 
«ent. of hydrogen into palladium. 
In conclusion, I wish to acknowledge my indebtedness to 
Dr Ludwig Mond for valuable advice and assistance in conducting 
these experiments, and to the managers of the Royal Institution 
for placing at my disposal the resources of the Davy-Faraday 
Laboratory. 
