on Low- Temperature Research, 1893-1900. 707 



bismuth and antimony, were almost certainly more apparent than 

 real ; an explanation of them lay ready to hand in the crystalline 

 structure of these bodies, the internal strains occasioned in which by 

 extreme lowering of temperature, would naturally result in the weaken- 

 ing of some set of cleavage-planes, and comparatively easy rupture. 

 Measurements of the elastic constant known as the " Young modulus," 

 showed that it increased between four and five times by cooling from 

 -f- 15° C. to — 182°, and balls of iron, tin, lead or ivory, rebounded 

 much higher after the same treatment when dropped from a fixed 

 height upon an iron anvil. The general outcome was to make it 

 clear that cohesion gains effectiveness with added contiguity of the 

 particles acted upon, in this resembling gravitation. Professor Dewar's 

 experiments with liquid air thus lent countenance to Lord Kelvin's 

 view that gravitation is adequate to account for cohesion. 



They also disclosed marked alterations, at low temperatures, in 

 the optical properties of certain bodies. Changes of colour, corre- 

 sponding to changes in the specific absorption of light, were at once 

 obvious. Vermilion and mercuric iodide paled from brilliant scarlet 

 to faint orange ; nitrate of uranium and the double chloride of 

 platinum and ammonium turned white — the original hue returning in 

 all cases with the restoration of warmth. Blues, however, remain 

 unaffected by cold, and organic dyes are but slightly sensitive to it. 



Temperature has long been known to play an important part in 

 the phenomena of phosphorescence ; their study, accordingly, under 

 the intensely frigid conditions realised by Professor Dewar's work in 

 liquefying intractable gases, seemed desirable. It yielded a variety 

 of most interesting facts. On the whole, bodies gain greatly in 

 phosphorescent capability by cooling to— 182° C. Gelatin, celluloid, 

 paraffin, ivory, horn, indiarubber — in all of which the quality is 

 ordinarily inconspicuous — emit a bluish luminosity, on being stimu- 

 lated by the electric light, after immersion in liquid oxygen. Alka- 

 loids forming fluorescent solutions invariably become phosphorescent 

 at low temperatures. Glycerin, sulphuric, nitric and hydrochloric 

 acids, and strong ammonia, are also very bright, as well as most sub* 

 stances containing a ketone group. Milk is highly phosphorescent, 

 pure water but slightly so. An egg shone as a globe of blue light ; 

 and striking effects were obtained from many other organic products 

 — from feathers, cotton-wool, tortoiseshell, p;iper, leather, linen, 

 sponge, besides some species of white flowers ; above all, from white 

 of egg, which, with proper management, became vividly self-luminous. 

 Complexity of structure was inferred to be one of the main conditions 

 upon which the possession of this quality depends. The discovery 

 that it belongs to oxygen alone among simple gases was accordingly 

 unexpected. A current of oxygen flowing into an exhausted tube, 

 after exposure to an electric spark, emits hazy white light, and the 

 attendant formation of ozone attests the simultaneous progress of 

 molecular change. The effect is completely stopped by the presence 

 of hydrogen, or by the least trace of organic matter. At the tempera- 



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