1910.] Precipitate Obtainable in Precipitin Interactions. 399 



present paper records the results of a gravimetric study of the reaction. 

 The weights of the two interacting bodies and the weight of the final 

 precipitate have been ascertained. The precipitin (anti-body present in the 

 serum of the immunised animal) cannot be directly determined by weighing, 

 as it forms only a small part of the dried antiserum. It will be assumed in 

 the paper to be proportional to the volume of the antiserum. The homologous 

 protein (either serum or egg-white) has been reckoned as milligrammes of 

 dried serum or dried egg-white. The precipitate formed in the interaction has 

 been weighed. 



The most striking feature of the results obtained is the strict proportion of 

 the weight of the precipitate to the amount of the antiserum, provided the 

 quantity of the homologous protein exceed certain minimal amounts. If a 

 quantity of dried horse serum such as 50 mg. be allowed to interact in suitable 

 dilution with 1, 2, 3 and 4 c.c. antiserum for horse serum the weights of the 

 precipitates will be in the ratio of 1, 2, 3 and 4. This fact stands in harmony 

 with the evidence obtained by Welsh and Chapman* concerning the origin of 

 the precipitate mainly from the antiserum. The quantity of precipitate 

 represents the " precipitable content " of the antiserum and its weight is 

 practically that of the precipitin present in the antiserum. 



Methods. 



Eabbits and cats were employed to produce the antisera. They received six 

 to eight injections of serum or egg-white into the abdominal cavity. The 

 quantity of material injected was determined by drying quantities of serum 

 or egg-white to constant weight. Approximately 1 gramme was given at 

 at each injection. The animals were killed by bleeding 12 to 16 days 

 after the final injection. The serum was allowed to separate spontaneously. 

 This serum was utilised at once for the experiments. The whole serum from 

 any immunised animal was mixed together before use, as it was found that 

 the weight of the precipitate from the antiserum first separated differed 

 considerably from that of the antiserum separated later. All vessels, glass 

 tubes, measures and pipettes were sterilised by steam immediately before use. 

 The saline solution (0'75 per cent. NaCl) was sterilised by boiling thrice on 

 successive days. The antiserum was measured by a pipette carefully 

 graduated by weighing the amount of mercury delivered. The sera and 

 egg-whites used as homologous proteins were similarly nlfeasured after suitable 

 dilution. The solid content of the serum or egg-white was estimated by 

 drying to constant weight a measured volume. In this way it was more easy 

 to ensure the absence of bacterial contamination. The quantities of the 

 * ' Roy. Soc. Proc.,' B, vol. 78, p. 297, 1906. 



