2 Scientific Proceedings, Royal Dublin Society. 



surface. In later experiments the process was slightly modified : 

 the sponge was transferred from a beaker of distilled water to a 

 dry weighing bottle, and its weight obtained direct. In the case 

 of sponges hirsute with hispidating spicules, the results thus 

 obtained would, from capillarity, be too high: these sponges were 

 therefore touched for an instant with a piece of filter paper before 

 being placed in the weighing bottle. The sponge was next dried 

 in the water oven, and again weighed. 



The specific gravity of the dried sponge being taken as 2 — and 

 this is a very close approximation — one-half its weight was deducted 

 from the first weighing ; the remainder is the weight of a volume 

 of water equal to the total volume of the sponge. The specific 

 gravity of flint varies from 2- 5 to 2*6. Taking the higher number, 

 we multiply that just obtained by it, and this gives the weight of 

 the total sponge supposed to be converted into flint. 



The sponge was next boiled in strong nitric acid till all traces 

 of organic matter were destroyed, and the spicules remaining 

 separated by filtration : after well washing they were transferred 

 to a platinum crucible, and the ashes of the ignited filter paper 

 added : after again drying, they were weighed (this weighing was 

 only made use of in the calculation when the weight of the dried 

 sponge was not obtained). Since the spicules contain about 7 per 

 cent, of water, they were ignited to eliminate this — first over a 

 Bunsen, and finally with a Herepath. The weight of the ignited 

 spicules gives the weight of the silica present in the sponge. By 

 dividing the number representing this by that representing the 

 weight of a mass of flint equal in volume to the whole sponge, we 

 obtain as a percentage the ratio of the silica present in the sponge 

 to that required to totally convert it into solid flint. An example 

 will render the process clearer. 



Sponge taken — Anthastra pyriformis, Sollas. 



Weight of the sponge full of water, 1*9203 grammes, 

 Weight of the sponge when dried, 0-788 grammes, 



1-9203 - (0-788 ± 2) = 1-5263 grammes, 



which is the weight of a volume of water equal to that of the 

 sponge ; this multiplied by 2*6, the specific gravity of flint, is 



