Quantities of foreign Matters diffused in the Atmosphere. 57 



sent difficulty of ascertaining chemically the causes of differ- 

 ences in the atmosphere, produced by the diffusion through it 

 of infectious and other matters in very minute proportions. 

 It would seem that the aqueous vapour likewise diffused 

 brings down with it when condensed, such miasmata, &c, 

 as were either merely suspended, or, being dissolved, have 

 much affinity for water. In fact we find the glass of win- 

 dows and other non-absorbent surfaces here incrusted with 

 salt, for three or four miles inland, after long southerly and 

 westerly winds; our stream of water from Dartmoor al- 

 ways acquires a slight saline impregnation before it reaches 

 the town; and in malaria and other unwholesome climates 

 the stranger is particularly cautioned to avoid the evening 

 dews. This natural dew, if collected in sufficient quantity, 

 would probably be most fully impregnated ; but when this 

 cannot be done, a common glass carboy, filled with a cheap 

 refrigerating mixture and suspended over a funnel in a gentle 

 current of air on a dewy evening, will collect a sufficient quan- 

 tity for experiment. Rain is too dilute, and the clouds from 

 which it falls too migratory to answer the purpose. For 

 matters little attracted by water, reagents of more energetic 

 affinities may be suspended in shallow vessels, kept cold, when 

 requisite, by refrigerating mixtures beneath. But when the 

 air is very dry, when it is necessary to concentrate upon a 

 very small quantity of reagent the foreign matter in a large 

 quantity of air, or to know the quantity of air submitted to its 

 action, the test may be contained in a tube like that of Liebig 

 for condensing carbonic acid, &c, in organic analysis, (de- 

 scribed below,) and the air blown through from a pair of small 

 cylindrical bellows, the cubical contents of which are known; 

 or for greater accuracy, by a condensing syringe, containing 

 an exact number of cubic inches ; though the bellows are 

 more conveniently portable. To thoroughly extract the 

 foreign matter, the air may be repeatedly passed through the 

 tube by receiving it at the end in a bladder furnished with 

 stop cock, &c, having a second bladder, 

 which can either replace it when full, and 

 removed to the entering end, or it may be 

 returned from one to the other, through a 

 tube with a two-way cock, to save the trou- 

 ble of screwing and unscrewing. 



Liebig' s condensing tube for carbonic acid, 



tyc, in organic analysis. — For carbonic acid 



the three lower buibs are filled just above 



the connecting tubes with solution of potass, through which 



it bubbles up and keeps the liquid in agitation; the two 



Third Series. Vol. 1 1. No. 64. July 1837. I 



