pBODtrcnos OF 



UNDULATOPvY FOKCES. HEAT. 



PERCUSSION is another source of caloric ; and of this 

 numerous instances are found in daily life. The cap uni- 

 versally employed in fire-arms, depends for its action on 

 the sudden evolution of heat by rapid percussion, pro- 

 duced by the hammer falling, and compressing the fulmi- 

 nating mercury which the cap contains. By hammering a 

 stout iron nail on an anvil, sufficient heat will be afforded 

 to ignite a match ; and at one period a similar plan was 

 adopted by artisans, for the purpose of obtaining heat to 

 light the fires of their forges. In the process of coining, 

 the sudden percussion or compression employed, produces 

 considerable heat. By suddenly compressing air in a 

 suitable vessel, sufficient heat may be obtained to ignite 

 a piece of tinder. The philosophical instrument-makers 

 supply a syringe for the purpose of illustrating this in- 

 teresting fact 



A change of bulk is generally attended with the produc- 

 tion or loss of heat, as a natural result of the contraction 

 or expansion of a body. Further remarks on this subject 

 will be found under the head of Latent Heat. 



CHEMICAL A'-TION is the source very generally resorted 

 to, for the purpose of affording heat for social and eco- 

 nomic purposes ; and combustion, as being the most im- 

 portant, demands special attention. In the ordinary 

 process of burning fuel, the natural heat contained therein 

 is called out by intense chemical action ; and just in pro- 

 portion a* this action is intensified, so is the amount of 

 beat increased. The draught of a tall chimney, of the 

 blast of the locomotive steam-jet, and of the fan or 

 pump of the smelting-furnace, each effect the rapid pro- 

 motion of chemical action between the oxygen of the air 

 and the charcoal, .fee. , contained in the fuel, and so produce 

 an intense combustion and great heat. 



The production of heat by chemical action, is so often 

 presented to the notice of the chemist, as to embarrass 

 our attempts to make a selection. The following experi- 

 ments, however, will tend to illustrate various instances 

 of theltind: 

 , i'mi< 1. Mix together two parts of chlorate of 

 potass with one part of powdered lump sugar, and drop 

 on the mixture a little strong sulphuric acid. The che- 

 mical action which ensues will cause the rapid production 

 of heat and light 



riment 2. Dissolve some phosphorus in bi-sul- 

 phide of carbon, and pour the liquid thus obtained on 

 some paper. In a few seconds the liquid will evaporate, 

 and the solid ]>tm;>honis, being left in a minute state 

 of division, will ignite, owing to the chemical action of 

 the air ; and sufficient heat will be produced to consume 

 the paper. 



i -immt 3. Mix suddenly equal volumes of ul- 

 phuric acid and cold water. The two liquids will 

 condense into a smaller bulk, and great heat will be 

 evolved. 



The addition of cold water to newly-made lime, affords 

 another instance of the production of heat by chemical 

 means. Amongst the bricklayers this plan is often used 

 for the purpose of heating their tea. It has been pa- 

 tented as a method of affording heat for domestic re- 

 quirements ; and to so large an extent is caloric disen- 

 gaged in this manner, that stables and outhouses have 

 been destroyed by fire, owing to heavy rain-storms moist- 

 lime which had been stored in them. 



SPOXTANIOUS COMBUSTION is often a dangerous source 

 of heat, and is caused by the chemical action of air on 

 oily substances, when these are in contact with fibrous 

 matter, such as cotton and hemp. If a small piece of 

 cotton wool is dipped into linseed oil, and then placed 

 under gentle pressure, it will generally inflame at the 

 nary temperature of spring or summer. From this 

 cause, the destruction of cotton-waste stores and mills 

 often takes place. This result is greatly promoted by a 

 minute state of division, one instance of which may be 

 i by trying the following experiment : 



Experiment 4. Introduce some dry tartrate of lead 

 into a narrow glass tube, having one of its ends sealed. 

 Heat the tube until no more acid vapours pass off, and 

 then instantly seal the open end. If the tube i broken, 

 w)in quite cold, the fine particles of the lead will inflame 



on coming into contact with the air, chemical action being 

 the cause of the heat thus produced. 



The most intense heat which can be produced by che- 

 mical means, is that afforded by the combustion of a 

 mixture of coal gas or pure hydrogen, and oxygen gases. 

 This is generally effected by means of a peculiar form of 

 blow-pipe, so arranged as to prevent risk of explosion. 

 As this will be fully described in the chemical section, we 

 shall not further allude to it here than by stating, that by 

 these means the most refractory metals may be melted, 

 and the earths fused : there are few solid substances 

 which cannot by its agency be reduced to a fluid state, or 

 be dissipated in vapour. 



ELECTRICITY is another source of heat, to which special 

 attention will be called in the proper place. By means 

 of electricity, produced by chemical action, the most 

 powerful effects of heat are afforded. 



VITAL ACTION is a source of heat with which all 

 are familiar ; and it seems chiefly to depend on the slow 

 combustion which the food .undergoes, after being ab- 

 sorbed by the stomach, and conveyed to the surface of 

 the lungs, in the form of highly carbonaceous blood. The 

 charcoal contained in the blood undergoes combustion by 

 coming in contact with the inspired air; and in every 

 respect the process is analogous to that which is observed 

 in the burning of coals in the ordinary fire-place. 



The intelligent reader will not fail to remark, that in 

 every source of heat which has been enumerated, motion 

 has been the leading cause ; and a general statement may 

 be made, that whenever motion is produced in matter, 

 heat is simultaneously evolved. It is tnie that daily ex- 

 perience does not tend to confirm such a proposition ; but 

 that is solely because the senses, and our means of mea- 

 suring heat thus evolved, are far too gross to permit its 

 detection. Heat, Light, Electricity, and Magnetism are 

 mutually producible : all matter seems to contain these 

 forces ; and motion, at any time, is sufficient to bring them 

 from a latent to a sensible state. 



THE EFFECTS OF HEAT. EXPANSION. 



HAVING spoken of the various sources of heat, we must 

 next enquire as to its effecte on matter generally : 

 and it is necessary to bear in mind, that any results to 

 which we shall allude, are produced, not in the atoms of 

 which a body is composed, but rather around them, mo- 

 difying and changing their relative positions. Many 

 errors are often fallen into with respect to this point ; and 

 nothing is more injurious to true progress in scientific 

 study, than a careless appreciation of the lessons and laws 

 which it teaches. 



There are but three states in which a body is found : 

 the solid, the liquid, and the gaseous or as vapour ; and, 

 as it has already been observed, these states are modifi- 

 cations of each other, effected by the presence or compa- 

 rative absence of heat. According to the generally re- 

 ceived doctrine, it is assumed that a solid is so, because 

 its particles are near to each other, or in close approxima- 

 tion that a liquid is produced from a solid by the addi- 

 tion of heat, which has the effect of removing the parti- 

 cles of a mass from each other ; and that by still further 

 carrying on the process, a gas or vapour is formed, 

 wherein the separation of particles is still more effected. 



It would thus be seen that heat has what is called a 

 repulsive power : in other words, it can lessen the cohesion 

 of particles, and so enlarge the bulk they occupy in mass. 

 As we shall presently find, the addition of heat almost 

 always increases the size of a mass ; and hence has arisen 

 the idea that it repels the constituent particles of that 

 mass. When, however, a body has changed its state by 

 this expansion, other laws govern its further increase in 

 bulk. Indeed, there is a special law of expansion inci- 

 dent to solid*, liquids, and gases ; and these considerations 

 are of the utmost importance in physical science and ita 

 applications. 



The expansion of solid bodies varies considerably on 

 their receiving an equal addition of sensible heat ; and 

 they expand in all directions, although differing in 

 individual extent. A very simple way of showing the 



