188 SCIENCE IN SHORT CHAPTERS. 



On my part, I repeated with further argument what I 

 have already urged on page 113 of the " Gentleman's Maga- 

 zine" for July, 1880, viz., the primary necessity of syste- 

 matic experimental investigation of the rate of exosmosis 

 (oozing out) of the gas from balloons made of different ma- 

 terials and variously varnished. 



Professor Graham demonstrated that this molecular per- 

 meation of gases and liquids through membranes mechani- 

 cally air-tight, depends upon the adhesive affinities of par- 

 ticular solids for other particular fluids, and these affinities 

 vary immensely, their variations depending on chemical 

 differences rather than upon mechanical impermeability. 

 My project to attach captive balloons, of small size to the 

 roof of the Polytechnic Institution, holding them by a steel- 

 yard that should indicate the pull due to their ascending 

 power, and the rate of its decline according to the compo- 

 sition of the membrane, was heartily approved by Mr. 

 Powell, and, had the Polytechnic survived, would have 

 been carried out, as it would have served the double pur- 

 pose of scientific investigation and of sensational advertise- 

 ment for the outside public. 



If the aeronaut were quite clear on this point could 

 calculate accurately how long his balloon would float he 

 might venture with deliberate calculation on journeys that 

 without such knowledge are mere exploits of blind daring. 



The varnishes at present used are all permeable by hy- 

 drogen gas and hydrocarbon coal-gas, as might be expected, 

 a priori, from the fact that they are themselves solid hy- 

 drocarbons, soluble in other liquid or gaseous hydrocarbons. 

 Nothing, as far as I can learn, has yet been done with sil- 

 icic or boracic varnishes* which are theoretically imperme- 

 able by Ifydrogen and its carbon compounds; but whether 

 they are practically so under ballooning conditions, and 

 can be made sufficiently pliable and continuous, are ques- 

 tions only to be solved by practical experiments of the 



* Since the above was written I have made some experiments with 

 a solution of shellac in borax (obtained by long boiling), and hereby 

 claim the invention of its application to this purpose, in order to 

 prevent anybody from patenting it. I shall not do so myself. 



