1906.] Main Source of " Precipitable" Substance, etc. 299 



alongside. In any one experiment it is possible to compare and classify the 

 precipitates with accuracy, and in different experiments or in different stages 

 of the same experiment a reliable comparison between the deposits may be 

 made by taking sufficient intervals between the readings such as might 

 be represented by traces, - 5 mm., 1 mm., 2 mm., and other multiples of 1 mm. 



Ordinary test-tubes and samples of the small test-tubes commonly employed 

 in precipitin reactions we found to be unsuited for the reading of the smaller 

 precipitates. Consequently, we Were obliged to make our own tubes from 

 narrow glass tubing not exceeding 4 mm. in diameter, the melted ends being 

 simply allowed to fall together in the flame. For special purposes tubing of 

 about 5 mm. diameter was employed. 



The following experiments afford a comparison between the precipitates 

 obtainable from the interaction with precipitin antisera of varying quantities 

 of proteid in solution and those obtainable from corresponding proteid 

 solutions by addition of proteid precipitants. 



In Experiment I two corresponding series of tubes were prepared containing 

 from O05 to O00000005 gramme dried horse serum in 0"5 c.c. saline solution 

 (0*75 per cent, sodium chloride), so that the dilution of dried horse serum 

 in 0'5 c.c. saline varied from 1 in 10 to 1 in 10,000,000. A control tube 

 containing - 5 c.c. saline alone was also prepared. To each tube of the first 

 series and to the control was added 0'01 gramme dried antiserum (prepared 

 in a rabbit by injection of dried horse serum) previously dissolved in 0*5 c.c. 

 saline solution. To each tube of the second series was added 05 c.c. of a 

 10-per-cent. solution of trichloracetic acid. The precipitates were read after 

 48 hours, as shown in Table I. 



Table I. — Primary Interactions. Experiment I. 



No. of 

 tube. 



Grammes of 

 dried horse serum 

 in each tube. 



Dilutions of 

 dried horse serum 

 in 0'5 c.c. saline. 



Precipitates in 

 tubes to which 

 0'01 gramme dried 

 antiserum was 

 added. 



Precipitates in 

 tubes to which 

 0'5 c.c. of 10-per- 

 cent, trichloracetic 

 acid was added. 









mm. 



mm. 



1 



0-05 



1 in 10 



1 



60 



2 



0-005 



1 „ 100 



1 -5 



21 



3 



0-0005 



1 „ 1,000 



1-5 



4 



4 



-00005 



1 „ 10,000 



1 -5 



1 -5 



5 



-000005 



1 „ 100,000 



0-5 



None 



6 



-0000005 



1 „ 1,000,000 



0-3 



None 



7 



-00000005 



1 „ 10,000,000 



0-3 



None 



8 



Control 



Saline alone. 



None. 





Hence, in Tubes Nos. 5, 6, and 7 definite small precipitates are given by 

 precipitin reactions in which the total amount of homologous proteid is 



