66 



KNOWLEDGE. 



[March 1, 1894. 



outside the central chamber. The outer annular chamber 

 or compartment was filled with ice to protect the next 

 compartment, also filled with ice, from the warming effects 

 of the external air. All the heat received by the inner 

 ice compartment comes, therefore, from the body placed in 

 the central chamber. The weight of ice melted measures 

 the quantity of heat received, the water formed by the 

 melting of the ice being run off from the bottom of the 

 apparatus, which is funnel-shaped and provided with a 

 stop-cock for the purpose. The above arrangements 

 served for the comparison of the specific capacities for 

 heat of different materials, as shown by the heat given out 

 by them in cooling through a certain range of temperature. 



In the experiments on the heat given out in burning 

 and in respiration, the arrangements were slightly different. 

 We give Lavoisier's description, retaining the old term 

 caloric, which may be taken to mean " heat considered as 

 a measurable quantity." Lavoisier writes : "To determine 

 the quantity of caloric disengaged during combustion and 

 during animal respiration, the combustible bodies are 

 burnt, or the animals are made to breathe, in the interior 

 cavity, and the water produced is carefully collected. 

 Guinea-pigs, which resist the effects of cold extremely well, 

 are well adapted for this experiment. As the continual 

 renewal of air is absolutely necessary in such experiments, 

 we blow fresh air into the interior cavity of the calorimeter 

 by means of a pipe destined for that purpose, and allow it 

 to escape through another pipe of the same kind ; and that 

 the heat of the air may not produce errors in the result of 

 the experiments, the tube which conveys it into the 

 machine is made to pass through pounded ice, that it may 

 be reduced to 82° before it arrives at the calorimeter. 

 The air which escapes must likewise be made to pass 

 through a tube surrounded by ice included in the interior 

 cavity of the machine, and the water which is there 

 produced must make a part of what is collected, because 

 the caloric disengaged from the air is part of the product 

 of the experiment." 



By means of this apparatus, or " machine" as he calls it, 

 Lavoisier compared the quantities of heat evolved during 

 the burning of a given weight of carbon and of hydrogen, 

 and of a given weight of animal and vegetable substances 

 formed of carbon and hydrogen, such as wax in a wax 

 taper, and olive oil burnt in a little lamp. He also, as 

 we have seen, determined " the quantity of caloric dis- 

 engaged during respiration," a research connected with 

 that on the loss of weight during respiration, in which his 

 colleague Seguin was the corpus vile in place of the guinea- 

 pig, which was found so "well adapted " for calorimetry. 

 Seguin used to be sewn up in a varnished au'-tight silk 

 bag, the edges of which were accurately cemented round 

 his mouth, leaving only a slit for breathing. He was 

 weighed in a delicate balance from time to time. 



Bince the days of Lavoisier thermo-chemistry, or the 

 study of the heat-changes which accompany changes of 

 chemical composition, has made considerable progress in 

 spite of the many practical difficulties which surround the 

 subject. One of the most important of the laws which 

 have been experimentally established is that " the initial 

 and final stages of a chemical reaction alone determine the 

 amount of the heat change." 



For instance, the conversion of a given weight of carbon 

 to carbonic acid is accompanied by the evolution of a 

 quantity of heat which is the same whether the carbon 

 be burnt rapidly in oxygen, or whether in a slow and 

 roundabout series of chemical changes the carbon is 

 successively a constituent of a number of vegetable and 

 animal substances before finally attaining the form of 

 carbonic acid in the expired breath of animals. This law, 



which has been experimentally proved to hold in a variety 

 of cases, enables us to jalculate the heat-giving power of 

 foods without having to ascertain the various changes and 

 modifications which the food undergoes in the animal 

 body. We know the final products, and that is sufficient. 

 Thus, sugar taken into the body is sooner or later 

 completely converted into carbonic acid and water ; in 

 other words, it is completely burnt. The heat given 

 to the body by one ounce of sugar is therefore, by the 

 second law of thermo-chemistry as it is called, the same as 

 the heat evolved by burning an ounce of sugar. This 

 quantity can be experimentally determined by the use of 

 Lavoisier's calorimeter, or one of the modern improvements 

 upon his original apparatus. We are thus enabled readily 

 to compare the heat-giving power of dift'erent foods and 

 food stuff's, and it is by the use of such thermo-chemical 

 methods that the figures quoted in previous articles have 

 been ascertained. For instance, the starch equivalent of 

 fats, two-and-one-third, is the proportion which the heat- 

 giving power of fats bears to that of an equal weight of 

 starch. 



Every chemical change is accompanied by a heat change, 

 but it must not therefore be supposed that every chemical 

 reaction is accompanied by an evolution of heat. There 

 may equally well be a disappearance of heat. It is the 

 former class of reaction, those in which heat is evolved, 

 which are often accompanied by striking phenomena such 

 as evolution of light, &c. Hence the popular idea that a 

 chemical change is necessarily accompanied by an evolu- 

 tion of light and heat. By the second law of thermo- 

 chemistry the initial and final stages only determine the 

 heat reaction. Take the case of the carbonic acid of the 

 atmosphere, and the various changes the carbon undergoes 

 after its assimilation by the green portion of a plant and 

 during its subsequent changes in the body of some animal 

 which has fed upon the plant. The final stage is that the 

 carbon is restored to the air as carbonic acid. The initial 

 and final stages are identical and the total heat reaction 

 is nil. While the carbon has been in the animal body it 

 has been gradually oxidized up to carbonic acid, and 

 during the whole of this time heat is being evolved ; hence 

 the warmth of the animal. The heat thus given out by 

 the carbon taken into the body as vegetable food 

 is (by the second law of thermochemistry') exactly 

 equal to the heat ahsorhed during the process of converting 

 the carbon of carbonic acid into the state of chemical 

 eombmation in which it is found in the plant. How is it, 

 then, that the frigorific effects of plant growth are so much 

 less patent and obvious than the heating effects of animal 

 life ■? It is mainly due to the circumstance that, in order 

 to bring about chemical changes in which there is an 

 absorption of heat, some external agent must act, and 

 must keep on acting, in order that the chemical reaction 

 may proceed, and in the case of the decomposition of 

 carbonic acid in the presence of the green colouring 

 matter of plants this external agent is the sun's rays. 

 Hence, the cooling eff'ect which results from the feeding of 

 plants upon their atmospheric food is to a great extent 

 masked by the fact that the cooling action only takes 

 place in presence of a potent heating agent — the sun's 

 rays. Still, let anyone compare the climate of a forest 

 and of a sandy desert in the same latitude, and reflect 

 that the difference is partly due to the fact that in the 

 one case all the heat rays are reflected back, but in the 

 other case a part of these rays are employed, without 

 apparent heating eft'ect, in splitting up carbonic acid. 

 This gives some idea of the cooling action of plant life, 

 which contrasts so sharply with the warmth of living 

 animals. 



