SPONGES AND SILICA 157 



Prodigious, no doubt ; but if we assume that the sponge 

 takes five years to grow from the larval stage to maturity 

 a mere guess suggested by what is known of the rate of 

 growth of the bath-sponge * then the average quantity 

 per year required to flow through the sponge is 200 litres, 

 and this will be found to work out to something less than 

 half a cubic centimetre per minute ; thus the prodigy is 

 reduced to quite commonplace dimensions. How gentle 

 such a flow would be may be judged from the following 

 considerations. Let us suppose the water to enter through 

 little pores in the outer surface of the sponge, as in fact it 

 does, and let us further suppose for the sake of calculation 

 that these pores are restricted to the four upright sides of 

 our prism, or an area of 7,000 square millimetres. Take 

 half this to represent the area of the pores, then the rate 

 at which the water would be flowing into the sponge 

 would be O'll of a millimetre per minute, or a little 

 slower than the movement of the hour-hand of an 

 ordinary watch. We have said our prodigy becomes 

 commonplace. Let us recall that word ; there is nothing 

 commonplace in Nature. And even if we reserve our 

 wonder for the immediately unexplained, we may well 

 marvel at the subtle chemistry by which, out of the 

 various constituents of sea-water, that living laboratory, 

 the sponge, is able to extract a single one, and that one 

 of the least abundant, in a state of almost absolute purity. f 

 In connexion with the subject of time, the question 

 naturally arises as to the period which would be required 

 to furnish the silica for a single flint. If we suppose the 



* It is found by observation that a bath-sponge requires from five 

 to seven years to grow from a fragment one cubic inch in bulk to the 

 " size of a saleable sponge." 



f It is said that our chemists are about to rival this process by 

 extracting in paying quantities the gold of sea-water, a rarer con- 

 stituent even than silica. 



