408 



KNOWLEDGE 



[May 15, 1885. 



air could have eflected so remarkable a diminution of the 

 conducting power of the air." 



The following is one of the experiments he made in 

 attacking this problem. He took a cylindrical clear glass 

 bottle, about 8 inches iu diameter and 12 inches high, 

 with a .short and narrow neck, suspended in the middle of 

 this a small piece of wetted linen rag ; then plunged the 

 bottle in a vessel of water at about 100" Fahr. It 

 remained there until the contents of the bottle were not 

 only heated, but thoroughly saturated with moisture by 

 evaporation from the rag. The cork was then taken out, 

 the rag removed, the bottle recorked immediately, and then 

 plunged into clear ice cold water contained in a cylindrical 

 glass jar. The apparatus being placed in a good light, 

 careful observations were made. 



The air in the bottle, being saturated with vapour of 

 water, it could not be cooled in any part without, " at the 

 same moinent and in the same place, parting with some of 

 its water " ; therefore, if the heat passed out from the air 

 in the bottle by penetrating the mass from the centre to 

 the surface by conduction from particle to particle, the 

 greater part of the air in the bottle would part with its 

 heat when not actually in contact with the glass, and a 

 proportional part of its water being let fall at the same 

 time and in the same place, would necessarily de.scend as 

 tine rain, probably invisible as rain, or as drops of water 

 on the bottom of the bottle, but would at least be visible 

 as " that kind of cloudy covering which cold glass acquires 

 from contact with hot steam or watery vapour ; but if 

 the particles of air, instead of communicating their heat 

 from one to another, from the centre to the surface of the 

 bottle, each in its turn and for itself, came to the surface 

 of the bottle, and there deposited its heat and its water," 

 he concluded that the amount of condensation on the sides 

 of the bottle should be as great as on the bottom, or even 

 greater. 



He found this to be the case, and further that it made 

 its first appearance upon the sides and near the top of the 

 bottle, and gradually spread downwards, " till growing 

 fainter as it descended lower, it was hardly visible at the 

 distance of half-an-inch from the bottom of the bottle ; and 

 upon the bottom itself, which was nearly flat, there was 

 scarcely the smallest appearance of cloudiness." 



Rumford's explanation of this is the same as is now 

 universally accepted, viz., that the particles or film of air 

 in contact with the glass is cooled, and deposits there some 

 of its moisture ; then, in consequence of its increased specific 

 gravity, it slides downwards by the sides of the bottle to 

 the bottom, where it takes its place and forces the whole 

 mass of warmer air upwards. This upper warm air is 

 cooled and descends in like manner, the deposition and 

 circulation continuing until all is cooled down to the tem- 

 perature of the surrounding water ; the upper part being 

 always the warmer during the action, more water is 

 deposited on the upper part of the sides of the bottle. 



Rumford's conclusion from this and other experiments 

 was that " though the particles of air, individually or each 

 for itself, are capable of receiving and transporting heat, 

 yet air in a quiescent state, or as a fluid whose parts are at 

 rest with respect to each other, is not capable of conducting 

 it or giving it a passage ; in short, that heat is incapable of 

 passing througli a mass of air, penetrating from one particle 

 of it to another." 



Here, then, we have an explanation of the mystery above 

 stated. The fine fibres distributed loosely through the air 

 in the globe of the passage thermometer prevented free cir- 

 culation of the air, for although so small a quantity, even 

 if absolutely non-conducting, could have had but little 

 eifect in preventing the air from "giving a passage to the 



heat " itself, it might, and must, very much interfere with 

 that circulation of the air which is necessary for the trans- 

 mission of heat by the " transporting " action (as Rumford 

 then called it) of the travelling particles, i.e., by what we 

 now call "convection currents." 



Here, then, we have the first step towards an explana- 

 tion of the protective power of the fur and feathers of 

 animals and the artificial clothing of man. Air, as Rum- 

 ford then contended, and as now is fully proved, is almost, 

 if not quite, a non-conductor of heat. Heat cannot pass 

 through air in the same way as it passes through metals, 

 but may be conveyed by the movements of the air itself. 

 The efi'eotive clothing agent is the nonconducting layer or 

 outer skin of air which envelopes our bodies and is held in 

 its place by the fibres of the clothing. The formation of 

 convection currents being impeded, we obtain the |)rotective 

 benefit of this non-conducting (or ill-conducting) skin of 

 entangled or imprisoned air. 



This is not only the case with the clothing of animals, 

 but also with the winter clothing of the earth. The feathery 

 crystals of ice of which snow is composed imprison a layer 

 of air many times greater in bulk than themselves, and 

 thus in climates where the winter temperature of the air 

 falls greatly below the freezing-point, the snow-clad ground 

 and its vegetation remain all the winter through at about 

 the freezing-point. 



We now apply the same principle to ice-safes, which 

 have double walls with powdered charcoal or sawdust 

 between them. The use of double windows, both for 

 retaining artificial heat in winter and resisting the passage 

 of outer heat in summer, depends on the same principle. 

 Doubling the thickness of the glais would have but small 

 eflect ; but the thick layer of air confined between them is 

 (if shaded from direct solar radiation) more efiective than 

 an equal thickness of solid masonry. Such a wall of air is 

 still more effective if its circulation is impeded. Thus 

 Rumford found that an amount of heat which passed 

 through a wall or sheet of confined air, h in. thick, in 9f 

 minutes, required 21-j minutes to make its way through the 

 same wall when the circulation of the air was impeded by 

 mixing it with J,.- of its bulk of eider-down, or very fine 

 fur, or of tine silk as spun by the worm. 



I should add that, at the time of making these experi- 

 ments, Rumford supposed, in common with the other philo- 

 sophers of the same date, that water and other liquids were 

 good conductors of heat, but that shortly after he refuted 

 this by following up the results of his experiments on the 

 transmission of heat by air. 



PLEASANT HOURS WITH THE 

 MICROSCOPE. 



By Henby J. Slack, F.G.S., F.R.M.S. 



IN Knowledge for March 20 reference was made to 

 Carter's box of seeds for the microscope, and some 

 of the most interesting from the Composite family were 

 described. The beauty and curiosity of seeds under the 

 microscope depends chiefly upon their integuments and 

 appendages. Their minute anatomy and physiology are 

 important and interesting studies, but we shall now con- 

 sider only their external aspects. 



Amongst the Umbellifera;, carrot-seed will be found to be 

 of oval shape, decorated with longitudinal ribs and pro- 

 jecting spines. Carraway-seeds are rich brown in colour, 

 with lighter ribs and no spines. The Spanish Balm 

 (Anethum) has also a ribbed seed, slightly hollowed like a 

 boat. Amongst the Scrophularians, which comprehend 



