440 Dr. Wall on the Ph^emmena 



free accefs of air, becaufe air is requiiite to 

 the formation, and, perhaps, as well as water 

 naakes a conftituent part of every perfecft cryftal. 

 If the furface of the water have not a free com- 

 munication with the air, and the boiling be 

 carried on rapidly, the fait falls down in fmall 

 granules, and no cryftals are formed. This 

 appears to me to be the efifeft of the thin oily 

 film in the procefs above defcribed, which is 

 explicable upon the principles formerly ad- 

 vanced. 



Of the effed of oil in fmooching troubled 

 waters, fo full an account is given by Dr. Frank- 

 liny in the Philofophical Tranfadlions for 1775, 

 that it is not neceffary to be particular as to 

 the matter of fadl, which is now generally 

 known. I think this faft alfo is eafily explicable 

 upon the principles, which I have laid down, 

 viz. that the particles of oil have a ftronger at- 

 tradion for each other (or inter fe J than they 

 have for water, and probably, than they have 

 for air. Air, we know, has a confiderable attrac- 

 tion to water, fo that the one is feldom free 

 from the other, and, when they are brought 

 into contad, they feem to unite and adhere by 

 the double force of chemical affinity and me- 

 chanical cohefion. Therefore, when a con- 

 fiderable body of air is forcibly impelled, as 

 in a dorm, upon the furface of water, it in a 

 manner lays hold of the water, carrying or forcing 



it 



