ON THE MIGRATION OP SWALLOWS. g J 7' 



in which indeed the elementary ingredients j^enerally acquire 

 a perfectly different arrangement, in every case of combina- 

 tion with other substances. 



I shall just add a remark, which I have often made, that Oxipen essen- 



substances not containing: oxijjen are perfectly mild in their ''^' 'o a^ri- 



• -1 • , • 1 mony. 



nature, and only acquire acrid or virulent properties when 



oxigen is added to them. Thus oxigen occasions the acidity, 



the alkaline property, and, as has been remarked to me, 



the causticity of other substances. To this ingredient all 



mineral remedies owe their powers, to this the metallic and 



some other poisons owe their deleterious properties. 



If the above observations deserve to occupy a page or two 



in the valuable Philosophical Journal, their insertion will 



much oblige. Sir, 



Your obedient servant, 



MARSHALL HALL, 



Member of the Royal Medical Society, 



University of Edinburghf 

 Sept. 21, 1810, 



vin. 



On the Migration qfSwallows, By Thomas Forster, Esq, 

 In a Letter from the Author. 



To Mr. NICHOLSON. 

 SIR, 



JLN a former number of your Journal you were so good as Remarks on 

 to insert my table of the times of appearance and departure f's migration 

 of several of our migratory birds; if you should think the 

 ensuing observations on the swallow tribe worthy of notice 

 and insertion, they are very much at your service. 



The swift, hirundo apus, which abounds in the neigh- Swift. 

 bourhood of Hackney, and annually builds in great num- 

 bers in the old steeple, was not seen at Hackney after 

 August the 13th ; and several days previous to this its num- 

 bers were greatly decreased. On the 17th I happened to 



be 



