420 Mr. J. Gill on the Change of State as 



as it would only withdraw one-half of the nitrogen that remained 

 after the first operation, a third dialysis one-half of the nitrogen 

 remaining after the second operation, and so on — each step of 

 the concentration of the oxygen being obtained at a greater cost 

 than the last, and the best conceivable result being only a good 

 approximation. The practical problem which is suggested by 

 the air-dialyzer is, to attain the means on a large scale of redu- 

 cing to one-half, or so, the proportion of nitrogen in atmospheric 

 air, to be applied to certain useful purposes. 



6. Percolation of air through gutta percha and other septa. 



Thin transparent sheets of a certain material represented as 

 air- and water-tight are in common use. It is often spoken of 

 as consisting of caoutchouc, but appears to have a body of gutta 

 percha, softened probably by a drying-oil. From its softness 

 and thinness, this sheet of gutta percha appeared at first highly 

 promising. But it appears not to be free from small apertures 

 for any considerable surface. When a small sound portion was 

 operated upon, air was found to percolate through it very slowly. 

 In a tube diffusiometer of 1*3 metre in length and 20 millims. in 

 diameter, closed at the top with this septum supported by stucco, 

 the mercurial column fell from 28*7 to 22*625 inches in 18-J 

 hours. The gas which had entered above the mercury measured 

 13'54 cub. centims., and was found to contain 20*2 oxygen to 

 79*8 nitrogen — a proof that the air had entered by gas- diffusion. 

 The material is in fact of sufficient porosity to permit the mole- 

 cular passage of gases in a slow manner. 



Varnishes of gelatine and of drying-oil have been tried as dia- 

 lytic septa, but hitherto without marked results. 

 [To be continued.] 



LVI. Note on Change of State as affecting Communication of 

 Heat. By Joseph Gill, Esq.* 



Y a train of analogies of a kind not admissible by our Ba- 

 conian system of philosophy, an Aristotle or a Kepler 

 might have dreamed of some physical law in virtue of which 

 matter in a lower state cannot, by direct contact, communicate heat 

 to matter of the same kind in a higher state though at a lower 

 temperature. For instance, ice at 32° cannot communicate heat 

 by direct contact to water overcooled to a lower temperature ; 

 and water under atmospheric pressure heated above its normal 

 boiling-point, which, under certain conditions, may easily be 

 done, cannot impart heat by direct contact to steam at the lower 



* Communicated by the Author. 



