OCCLUSION OF OXYGEN AND HYDROGEN BY PLATINUM BLACK. 
i.sr 
ments may thus amount to about one and a half per cent., whilst in the last three it 
might exceed seven per cent. 
If we take the mean of Experiments I., II., and III., and also include another 
independent result, 68'2 K, which was obtained incidentally during the determination 
of the heat evolved on the occlusion of oxygen by platinum black,wn find as the 
general mean value for the heat evolved per gram of hydrogen occluded, 68’8 K, 
or i37’6 K per gram-molecule; and we think that, with the liberal allowances 
we have made for experimental error, this number may be taken as correct within 
one or two per cent. 
According to Berthelot the hydrogen which cannot be pumped off at the ordinary 
temperatm’e from platinum black charged with hydrogen forms the compound PtgoHo, 
whilst that which does come off is obtained by the dissociation of PtgQHg into the 
first compound and hydrogen. 
The heats of formation of these hypothetical compounds are + 339 K for the first, 
and -r 426 K for the second; that is to say, 170 K are evolved per gram of 
hydrogen occluded in the first instance, and 87 K per gram in the second. From 
the results which we have obtained it follows that the arguments put forward by 
Berthelot in favour of the existence of these compounds cannot be justified. The 
second value for the heat of occlusion of hydrogen, which is only half the first, 
is still much higher than the real value, 68'8 K, and we can only account for these 
different and high numbers found by Berthelot on the assumption that the 
platinum black employed by him contained oxygen. If reference is made to 
Table Ill.t it will be found that when the platinum black contained oxygen, the 
following numbers w’ere found by us for the heat evolved per gram of hydrogen 
ahsorhed: 203, 195, 183, 173, and 163 K, which are of the same order of magnitude 
as that given by Berthelot for the supposed heat of occlusion of hydrogen in the 
first compound, viz., 170 K. 
Favre has attempted to distinguish between the heats evolved on the occlusion of 
hydrogen by platinum and by palladium, inasmuch as when hydrogen is admitted in 
small portions at a time to palladium, the heat of occlusion remains constant ; whilst 
in the case of platinum the heat evolved becomes gradually less and less. This 
difference, which however is only apparent, is also exemplified in Table III., and it 
is only necessary to point out that it is the result of adding hydrogen to platinum 
charged with oxygen, and therefore, in this respect, the supposed difference between 
the behaviour of platinum and palladium to hydrogen does not exist. 
* Page 14.5, Table HI, Operation 15. 
t Page 145. 
VOL. CXC.-—A. 
T 
