622 Proceedings of the Royal Irish Academy. 



■wMcli it seems hard to eliminate, arises from the fact that the effects 

 of the different gases may he more or less rapid. Thus it is very- 

 certain that the light gases {i.e. those with small molecules) will 

 diffuse into the intercellular spaces more quickly than the heavy gases, 

 and so come into contact with evaporating cells more quickly. 

 Besides this, it seems prohable that the poisoning and anagsthetic 

 effects of one may be more rapid than those of another. The 

 observations, on which the numbers given above are based, were 

 commenced in each case after the branch had been surrounded by the 

 gas for five minutes, and were discontinued before any lethal could be 

 observed in the leaves ; for such, if arising, would cause the osmotic 

 pressures obtaining in the leaf-cells to become diminished, by ren- 

 dering the protoplasmic membranes permeable. These effects were 

 usually visible within 45 minutes after starting the experiment. It 

 is possible that the denser vapour could not, within this time, diffuse 

 into all the intercellular spaces of the leaves. 



An error arising from this possibility is most unsatisfactory, 

 as it seems extremely difficult to make proper allowance for it. 

 It seems impossible, at present, to decide how soon the surrounding 

 gas will come into contact with the evaporating cells, and, 

 also, when the ansestheticising or stimulating action will cease, 

 and the lethal effects will begin, if, indeed, there is any sharp 

 line of distinction. 



Next we come to an error which can, in some degree, be elimi- 

 nated. 



It is known that the rate of diffusion of a gas will be influenced 

 by the nature of the gas occupying the space into which it is diffusing 

 Thus water-vapour will diffuse more slowly into CO2 gas than into 

 oxygen. This difference depends on the relative sizes of the molecules 

 of the gases into which the water-vapour has to diffuse. For the 

 same pressure and temperature, there will be the same number of 

 molecules of these gases in the surrounding space ; but if their sizes 

 are different, it is plain that the water-molecules will less readily 

 diffuse into the space occupied by the gas composed of the larger 

 molecules. 



In order to form some idea of this effect, I suspended a shallow 

 dish containing water in the receiver, previously occupied by the 

 transpiring branch ; and in connexion with the train of apparatus 

 previously described, successive weighings gave approximately the 

 loss of water by evaporation from this dish. During the experiment 

 a stream of gas dried, as before described, was kept up through the 



