xvi INTKODUCTORY NOTE. 



they manifest their extraordinary vitality in the body 



of the liquid itself. 



On public sympathy the sanitary physician has 

 mainly to rely for support, in a country where sanitary 

 matters are left so much in the hands of the public 

 itself as they are in England. But sympathy without 

 cause — that is to say, without some basis of knowledge 

 — is hardly to be expected. It is as a contribution to 

 such knowledge that these Essays have been collected, 

 and thrown into their present handy form. 



J. Tyndall. 



Royal Institution : 

 Aiiffust 1881. 



