350 Dr. W. Marcet on the Nutrition of 



sues, these inquiries relating more particularly to the nutrition 

 of muscles and lungs in health and when affected with phthisis : 

 and I must begin by acknowledging the valuable assistance of 

 Messrs. H. Bassett^ F. A. Manning, and M. J. Salter in the ana-= 

 lytical portion of the inquiry ; I am much indebted to these 

 gentlemen for the care they have bestowed on the work. 



The subject is treated by methods of investigation which 

 may be considered new; it is therefore necessary that I should 

 enter into their details, so as to make the mode of reasoning, the 

 analytical process adopted, and the results obtained equally and 

 thoroughly clear to the reader. By this means only can I hope 

 to forestall objections and establish the correctness of my work. 



I must beg leave to begin with a few introductory remarks 

 relating to liquid diffusion, a subject which has been so admira- 

 bly treated by Graham.. 



If w^e suppose a solution of common salt, on which a flat piece of 

 cork is floated, and if a stream of water be poured carefully upon 

 the cork, the water will not mix immediately with the solution 

 of salt, but form an upper layer in the receiver, while the solution 

 will occupy the inferior layer. Supposing no cause w^hatever 

 to agitate the fluids, that they be neither shaken nor subjected 

 to any current of air, they will, hov»^ever, undergo a tolerably 

 rapid process of mixing, the solution of salt moving into the 

 water, or, in other words, distributing itself throughout the water. 

 This phenomenon is called Liquid diffusion. 



The rate at which diffusion takes place varies according to the 

 substance in the solution ; hence it is said that different solutions 

 have different rates of diffusibility. Chloride of sodium may be 

 regarded as yielding aqueous solutions possessed of this property 

 in a very high degree ; while white of egg or blood allow of the 

 distribution of their albumen through water at a very slow rate 

 indeed. 



Now, supposing that a jelly be prepared, by dissolving isin- 

 glass in a weak solution of chloride of sodium (in a strong solution 

 the jelly may not set) . If distilled water be poured over this jelly, 

 the salt will by degrees find its way out of the jelly into the water, 

 and will continue doing so until it be distributed equally through- 

 out the jelly and the water. Should the water and the jelly occupy 

 the same bulk, we shall find, after a certain number of hours, the 

 same amount of chloride of sodium in the jelly and the water. 

 Should the volume of the water be twice that of the jelly, a cer- 

 tain bulk of the water, after complete diffusion, will only contain 

 half the amount of salt present in an equal bulk of the jelly, 

 and so on. On the other hand, if a solution of common salt in 

 water be poured over a jelly of gelatine, after a time the salt 

 will be found distributed throughout the water and jelly propor- 



