AIR. 



[ 23 ] 



AIR. 



flection taking place from its surface. So 

 long as air-bubbles or confined portions of 

 air are large, the optical appearances above 

 described are sufficiently characteristic ; al- 

 though should any doubt exist as to the 

 nature of a supposed accumulation of air, 

 the latter must be displaced, either by pres- 

 sure bet^veen two slips of glass, or by im- 

 mersing the object in which it exists in some 

 liquid and apphnng heat. When, however, 

 air is couiined within very minute cavities, 

 especially when these possess definite forms, 

 the clear centre is frequently no longer to 

 be detected, the whole appearing perfectly 

 black and solid; and serious errors have 

 arisen from inattention to this cii-cum- 

 stance, as explained in the Introduction 

 (p. xxxv). 



The corpuscles of dried bone were thus 

 formerly considered sohd bodies, as their 

 name implies, and as consisting of calcare- 

 ous matter, until it was found that they 

 could be filled with a liquid. In all cases, 

 tlieu, 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 pres- 

 sm-e or prolonged immersion in a liquid, 

 especially with the aid of gentle heat. 



The appearance presented by air con- 

 tained in tissues, is easily studied in a dry 

 section of any kind of pith or other vege- 

 table structure, such as elder-pith, rice- 

 paper, or cork ; cork is really heavier than 

 water, and owes its lightness to the air it 

 contains. On immersing these in water, 

 this liquid soon enters the lateral cells, but 

 long digestion is required before the in- 

 ternal cells become filled with it and the 

 whole of the air is displaced. Soaking in 

 alcohol before immersion in water greatly 

 facilitates the displacement of the air. 



The determination of the actual nature, 

 as regards chemical composition, of air con- 

 fined in tissues, is a matter of difficulty 

 where the quantities are microscopic. The 

 nitrogen can only be detected by its nega- 

 tive properties to reagents. The presence of 

 oxygen might be determined by moistening 

 a section of any structure with recently 

 boiled distilled water and then placing it in 

 a cell containing a solution of protosulphate 

 of iron, or an alkaline pyrogallate, and im- 

 mediately sealing the cell with varnish and 

 allowing the action to continue for some 

 time. 



For the detection of Caeboxic 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, and Rocks. 



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, mosses, and algas, pollen, 

 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, crystals, and 

 metallic particles, 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 Avith particles of carbon or soot, the 

 ova of the lower animal forms being com- 

 paratively 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 numerous. 



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, their resistance to com- 

 pression, and their not being destroyed or 

 decomposed by exposm'e to a red heat. 

 Certainty as to their composition can only 

 be obtained by chemical analysis. See 

 Sand. 



The organic 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 and fungi, according to 

 the nature of the decomposing matter, 

 being discovered in them in abundance. It 

 seemed very natura"l to conclude that these 

 derived their origin from the substances 

 undergoing decay ; and it is not to be won- 

 dered at, that the fact should have given 

 rise to the conclusion that here was evidence 

 of the spontaneous or equivocal generation 

 of animals. 



