AIR. 



[ 19 J 



AIR. 



sequence of the reflexion taking place from 

 its surface. So long as air-bubbles or con- 

 fined portions of air are large, the optical 

 appearances above described are sufficiently 

 characteristic; although should any doubt 

 exist as to the nature of a supposed accumu- 

 lation of air, the latter must be displaced, 

 either by pressure between two slips of glass, 

 or by immersing the object in which it exists 

 in some liquid and applying heat. When, 

 however, air is confined within very minute 

 cavities, especially when these possess defi- 

 nite forms, the clear centre is frequently no 

 longer to be detected, the whole appearing 

 perfectly black and solid, and grave errors 

 have arisen from inattention to this circum- 

 stance, as explained in the Introduction 

 (p. xxii). 



The corpuscles of dried bone were thus 

 formerly considered as solid bodies, as their 

 name implies, and as consisting of calcareous 

 matter, until it was found that they could be 

 filled with a liquid. In all cases, then, where 

 absolute certainty is required of the nature 

 of an apparent air-bubble or accumulation of 

 air, attempts should be made to displace the 

 mass, either by pressure, or prolonged im- 

 mersion in a liquid, especially with the aid 

 of gentle heat. 



The appearance presented by air contained 

 in tissues, is easily studied in a dry section 

 of any kind of pith or other vegetable struc- 

 ture, such as elder-pith, rice-paper or cork. 

 (Cork is really heavier than water, and owes 

 its lightness to the air it contains : see CORK.) 

 On immersing these in water, this liquid 

 soon enters the lateral cells, but long diges- 

 tion is required before the internal cells 

 become filled with it and the whole of the 

 air is displaced. 



Gases of whatever kind present the same 

 appearances under the microscope as air, 

 excepting those which are coloured ; but as 

 these are not naturally met with, they 

 require no notice. 



The determination of the actual nature, as 

 regards chemical composition, of air confined 

 in tissues, is a matter of difficulty, where the 

 quantities are microscopic. The nitrogen 

 can only be detected by its negative proper- 

 ties to reagents; the presence of oxygen 

 might be determined by moistening a sec- 

 tion of any structure with recently boiled 

 distilled water, and then placing it in a cell 

 containing a solution of protosulphate of 

 iron, and immediately sealing the cell with 

 varnish and allowing the action to continue 

 for some time. 



For the detection of CARBONIC ACID, see 

 that article. 



There is yet a source of fallacy in the 

 detection of air imprisoned in structures 

 where these are of a hard resisting nature, 

 as in mineral bodies. An illustration of this, 

 with the method of its avoidance, is given 

 under TOPAZ. 



In regard to the solid particles present in, 

 or subsiding from the air, and forming dust, 

 these consist principally of the spores of 

 fungi, lichens, algae and mosses, the detritus 

 of the soil, fine fragments of vegetable and 

 animal fabrics accidentally separated and 

 diffused during the ordinary operations of 

 every-day life, the dried, but not dead 

 bodies of infusoria, and the ova of the lower 

 members of the animal kingdom. The kind 

 of bodies present in the air varies according 

 to the locality ; thus in cities, the dust con- 

 sists mostly of fragments of products of 

 manufactures, with the spores of fungi, 

 mixed with particles of carbon or soot, the 

 ova of the lower animal forms being compa- 

 ratively few, and belonging to a limited 

 number of species ; whilst in open places in 

 the country, a more ready diffusion of the 

 spores of plants and the ova of animals takes 

 place, and the sources from which fragments 

 of textile fabrics are derived, are less nume- 

 rous. 



The inorganic particles deposited from the 

 air, consist of fine grains of sand, wafted 

 from the soil by winds, and rarely fall other- 

 wise than near the currents by which they 

 are borne. They are easily recognized by 

 their angular forms, then* resistance to com- 

 pression, and their not being destroyed or 

 decomposed by exposure to a red heat. 

 Certainty as to their composition can only be 

 obtained by chemical analysis. See SAND. 



The animal forms deposited from the air 

 formerly gave rise to much perplexity. It 

 has long been known that when solutions of 

 various organic substances, or liquids con- 

 taining these matters, undergoing sponta- 

 neous decomposition, were exposed to the 

 air, the liquids were soon found to teem 

 with life ; infusoria of various kinds, accord- 

 ing to the nature of the decomposing 

 matter, being discovered in them in abun- 

 dance. It seemed very natural to conclude 

 that these derived their origin from the sub- 

 stances undergoing decay, and it is not to be 

 wondered at, that the fact should have given 

 rise to the conclusion that here was evidence 

 of the spontaneous or equivocal generation 

 of animals. 



c2 



