POLARISED LIGHT. 



159 



He 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 knew 

 what has become of the various substances which he migut 

 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-fluid 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. 1 



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

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



i'l) A drop of a solution of Cinckonidine is better. See Addenda. 



