LXXXVIII Johan Gadolin. 



lands, das Impfen der Knhpocken seine Ärtzte und heisse Sehnsucht nach Frieden die Herzen 

 seiner Bewohner. 



Ihrem wohlwollenden Angedenken empfehle ich mich angelegentlichst, und verharre mit 

 unei'löschlicher Hochachtung 



Dero 



gehorsamster Diener 

 J. F. Omelin. 



A. Crawford an Joh. Gadolin. 



Dear Sir. 



I had great pleasure from the receipt of your obliging letter and shall be happy in 

 future to be favoured with your correspondence. The intercourse, which I had with Mr Ga- 

 dolin when he was in London, cou'd not I am sure be more agreeable to him than it was to 

 me. I have not a doubt but the experiments to which you allude would be highly deserving 

 of the attention of the Royal Society and I shou'd be much obliged to you, if you would dé- 

 sire Sir Joseph Banks to communicate them to me before they are read. 



The effect of a small quantity of air in promoting the formation of vapour is indeed 

 remarkable and deserves to be more particularly enquired into. I hope you have before this 

 time made some farther observations on that subject. 



I have lately made some experiments wich I think prove that the separation of 

 aqueous vapour from air produces cold, provided that the vapour be not Condensed. 



When I introduce equal quantifies of dry common air into the cylinders described in 

 my book and immerse them when hot into equal quantifies of cold water, the heats which 

 they communicate are equal. But when I introduce a dröp of water into one of the cylinders, 

 I find that this cylinder now containing damp air imparts a little less heat than that which 

 contains the dry air. If I introduce two dröps of the water into the same cylinder, a similar 

 effect is produced; but when I introduce four drops, this cylinder now communicates more 

 heat than the other. I cannot account for this except by supposing that in the first experi- 

 ments with the damp air the water chemically unités with the air when heated and again 

 separates from it when cold and that during this separation cold is produced. 



In conséquence of the proposais mentioned in your letter I applied to the Board of 

 Ordnance to send a chemical gentleman to collect spécimens in the North of England. To 

 this they have agreed. I shall therefore be much obliged to you, if you will send me a col- 

 lection of Swedish minerals as we have very few foreign spécimens. I shall have it to your 

 judgment to détermine the kinds which wou'd be most interesting and you will oblige me if you 

 will be so good and send duplicates of each spécimen, because the Duke of Richmond as well 

 as myself wishes for a collection of Swedish minerals. I shall not fail to send you such a 



Tom. XXXIX. 



