DIFFUSION OF HYDROCYANIC ACID VAPOR 187 
after standing over night it was as little as 0.66 per 
cent., averaging 4.71 per cent.; the particular amount 
was determined for each experiment and was deducted 
to determine the amount of actually liberated or avail- 
able hydrocyanic acid vapor. 
In the case of the room experiments the average 
residual hydrocyanic acid was found to be 4.97 per 
cent. of the whole amount; this was deducted in each 
case to determine the liberated or available vapor. 
Thus, at the temperature at which the experiments 
were made, both with large and small quantities of 
reagents, the proportion of acid and water used gave 
about 95 per cent. of available vapor. This liberated 
or available vapor on an average 0.07623 gramme per 
cubic foot of space, called by Professor Penny the 
““HCN,’’ was taken as the basis of calculation to 
determine the ratio of diffusion; in other words, that 
amount found in a cubic foot would be called a diffusion 
of 100 per cent. 
Quite as important as the average percentage of 
diffusion, Professor Penny shows, is the average per- 
centage of fluctuation. In such work absolute uni- 
formity of conditions with the facilities available is 
not attainable; hence repeated trials with conditions 
apparently the same often give different results, some- 
times widely different. As perfectly air-tight walls, both 
of boxes and of the room, are difficult to secure, there 
are varying conditions that make no two trials exactly 
alike. Such discrepancies, although they cannot be 
foreseen, may be explained by variations in the force 
and direction of the wind, varying amounts of humidity 
and the host of minor conditions that are never quite 
