96 WHAT IS LIFE 



of vitriol sulphuric acid to a piece of carbonate 

 of lime. There will be profuse effervescence, due 

 to the breaking up of the union between the 

 carbonic acid and the lime and the giving-off of 

 the former in the shape of gas. After a time this 

 will cease and all the carbonic acid gas having 

 been got rid of, sulphate of lime or gypsum will 

 remain in the receptacle in which the mixture was 

 made. Now all these things are explained by the 

 bringing together of the sulphuric acid and the 

 carbonate of lime, the former having a greater 

 affinity for the lime-base than the latter drives it 

 off and the operation is at an end. The train of 

 circumstances looks backward for its explanation. 

 In the case of arcella this is not so. It, too, 

 no doubt, produces gas, but not for the mere pur- 

 pose of producing it, nor for the purpose of unit- 

 ing two chemical substances with one another. It 

 forms gas because that is its means of overcoming 

 a problem in locomotion which has presented itself. 

 Hence the formation of the gas looks forward for 

 its explanation, in other words it is one which has 

 a telic explanation and looks towards an end and 

 that end a link in a chain of events, for the arcella 

 makes the gas, no doubt, in the first instance, in 

 order that it may turn right side up, but chiefly in 

 order that it may resume the even tenor of its 

 ways which have been temporarily interfered with 



