Testing dried bloods 67 



colourless, whereas when haemoglobin is present the solution may be 

 more or less coloured. I have attached no particular importance to the 

 amount of colouring matter which has gone into solution, but have 

 throughout judged a blood sample to have been soluble or not by means 

 of the foam-test. In earlier papers by Uhlenhuth, as also in those of 

 Ziemke, stress is laid upon a trace of colour in solution as an index 

 of the blood being in solution. In a later paper by Uhlenhuth (18, IX. 

 p. 679, 1902) that author for the first time mentions the foam-test, 

 which I have throughout my work considered essential in this respect. 

 I judge the solubility by blowing air gently through the pipette which 

 is used for transferring the solution into the test-tubes (Fig. 3 a). 

 If air is blown violently through the fluid containing the filter-paper, 

 particles thereof, as also the particulate matter in the blood attached to 

 the paper, are loosened and the solution is rendered cloudy. Solutions 

 of blood or serum of 1 : 1000 and over still foam well, the limit being 

 apparently reached with solutions of about 1 : 5000, when, in several 

 quantitative experiments, it was seen that the minute bubbles formed 

 " held " an instant longer than they did when air was bubbled simply 

 through normal salt solution. I find it possible, moreover, to tell 

 approximately the degree of concentration of a solution by the length 

 of time the bubbles remain at the surface after air has been bubbled 

 through the fluid. Even from a medico-legal point of view the colour 

 test of solubility as noted above may prove fallacious, for stains due to 

 serum, which we know contains the acting substance, might well be 

 overlooked in the absence of the foam-test to which I unqualifiedly 

 attach more importance. 



When the filter-paper squares are placed in salt solution, the serum 

 contained in the meshes of the paper usually goes into solution very 

 promptly, often in a few minutes. At times the solution is delayed 

 for half-an-hour or more. Once solution has taken place, it is inad- 

 visable to keep the paper soaking too long as the solid particles may in 

 turn become detached. Solution may be entirely prevented through 

 the paper having become soiled with fat, this having been the case 

 apparently with a few of the samples collected for me from animals. 

 In such cases the immersed paper tended to float, and remained covered 

 by minute silvery bubbles of attached air. 



As soon as solution has taken place, as evidenced by the foam-test, 

 the mixed solution is transferred to small test-tubes (Fig. 3 c) placed in 

 suitable racks. The test-tubes I used usually had a capacity of about 

 1 c.c. and were kept scrupulously clean, and were always well dried. 



5—2 



