140 On Rain Water. [Feb* 



Though the numbers which I have chosen for azote and its 

 compounds agree pretty well with experiment and with one another, 

 I am not quite satisfied with them : some mystery still hangs over 

 this intricate substance, which probably will not be removed till w© 

 become acquainted experimentally with its composition. 



(Tb be continued.) 



Article XI. 



On Rain IVater. By Mr. Stark. 

 (To Dr. Thomson.) 



Slit, 

 I HEREWITH send you extracts from a paper which I ha\'e re- 

 cently read at the Norwich Philosophical Society, and should you 

 think them worthy of a place in your Annals of Philosophy, they 

 are at your service. My motive for making them public is to in- 

 duce others vvho have more leisure for experimental research than 

 myself to make further inquiries into the subject. 



Your most obedient servant, 



KorwkJi, Dec. 15, 1S13. W.M. StARK. 



1 have long adopted the use of logwood (hamatoxylam campe- 

 chianum) in preference to litmus, or any other of tiie vegetable 

 colouring substances which are used as tests, finding it to be more 

 powerfully acted on both by the alkalies and the acids.* 



I was making some experiments on log\vood during a violent 

 thunder-storm, and exposed a few grains of it to tlie action of rain 

 Ivhich fell at that time ; the colour extracted was an orange red, 

 the same as is produced by distilled water slightly acidified. This 

 singular effect was so unexpected, that I doubted the accuracy of 

 the experiment : a short time afterwards I repeated it, under similar 

 circumstances, and found the same result. 



On exposing logwood to the action of rain which fell when there 

 had been no thunder for several weeks, a colour inclining to lilac 

 was extracted ; and water caught at the same time, at the height of 

 300 feet, produced also the lilac shade. These facts evidently 

 indicated either the presence of lime, or some kind of alkaline 

 matter. 



1 have frequently suspected the existence of such substances in 

 the atmosphere, from observing colours that had been mordanted 

 and dved precisely the same, would, by exposure to the air, be 

 sometimes reddened by the absorption of oxygen, or carbonic acid ; 

 at other times made bluer : this effect could not have been froia 

 any other cause than the action of lime, or an alkali. 



» Clievreul notices the delicacy of logwood as a (est, Ann. de Chim. toa 81, 



