246 Transactions of the Society. 



VI. — On a Method of Increasing the Stability of Quinidine 

 as a Mounting Material. 



By H. G. Madan, M.A. F.C.S., Fellow of 

 Queen's College, Oxford. 



{Read 20th March, 1901.) 



The allotropic colloid into which ordinary crystallised quinidine is 

 converted by exposure to a temperature slightly above its melting 

 point, has been tor some time used by microscopists as a very ex- 

 cellent mounting material for diatoms, &c, possessing as it does a 

 fairly high refractivity (yu. = 1'602 for yellow sodium light), and 

 giving exceptionally clear and brilliant images. The one great draw- 

 back to its use (apart from its brittleness) appears to be its instability, 

 or tendency to become opaque owing to its reversion to the crystalline 

 form. 



In a paper on mounting materials which I was kindly permitted 

 to present to the Society in April 1898,* I suggested two possible 

 reasons for this tendency to change : — 



(1) The presence of solid particles of dust, &c, which would serve 

 as nuclei and promote crystallisation. 



(2) Exposure to warmth, such as that of a hot room or sunlight ; 

 since at a temperature of about 90° C. the conversion of the colloid 

 (as hitherto obtained) into the crystalloid form is certainly rapid and 

 complete. 



Since then a third reason has occurred to me, which may very 

 probably be the principal one in determining the change, viz. insuffi- 

 cient duration of the heat employed in effecting the conversion into 

 the colloidal form. 



In the case of sulphur, which at temperatures above 150° C. 

 passes into the allotropic form known as " plastic sulphur," the change 

 is certainly associated with the transfer of a large amount of kinetic 

 energy (supplied in the form of heat) to the molecules of the plastic 

 substance, where it is stored up as statical energy (in the obsolete 

 phraseology of the older physicists, " a large amount of heat becomes 

 latent "). This is proved by the two following observations : — 



(a) That when ordinary sulphur is melted and the application 

 of heat steadily continued, the temperature of the liquid at first rises 

 uniformly by equal increments in equal times ; but at a point a little 

 above 150° a pause takes place, the temperature rising with extreme 

 slowness, most of the heat passing into some other form of molecular 



[* Journ. Roy. Micr. Soc, 1898, p. 273. 



