POLARISED LIGHT. 



Ii9 



lie says : " It has long been a favourite subject of inquiry 

 "with the professional man to trace the course of remedies 



Fig. 91.— Urinary Salts. 



a, Uric acid; b. Oxalate of lime, octahedral crystals of; c, Oxalate of lime 

 allowed to dry, forming a black cube; d, Oxalate of lime, as it occasionally 

 appears, termed the dumb-bell crystal. 



in the system of the patient under his care, and to kno v 

 what has become of the various substances which he might 

 have administered during the treatment of the disease. 



" Having been struck with the facility of application, 

 and the extreme delicacy of the reaction of polarised light, 

 when going through the series of experiments upon the 

 sulphate of iodo-quinine, I determined upon attempting 

 to bring this method practically into use for the detection 

 of minute quantities of quinine in organic fluids ; and after 

 more or less success by different methods of experimenting, 

 I have at length discovered a process by which it is possible 

 to obtain demonstrative evidence of the presence of quinine, 

 even if in quantities not exceeding the one-millionth part 

 of a grain ; in fact, in quantities so exceedingly minute, that 

 all other methods would fail in recognising its existence. 

 Take for test-Jiuid a mixture of three drachms of pure 

 acetic acid, with one fluid-drachm of rectified spirits-of- 

 wine, to which add six drops of diluted sulphuric acid. 



" One drop of this test-fluid placed on a glass-slide, 

 and the merest atom of the alkaloid added, in a short time 



