May 25. iJ^93] 



NATURE 



89 



that regulation is insufficient. In winter time we use thicker 

 clothing, we need more food, and if the cold is very great, we 

 produce more heat by muscular action. In accordance with 

 that experience, I found that animals produce more heat in 

 winter than in summer. If nourished with the same food, suffi- 

 cient to maintain their weight constant in winter, they do not 

 oxidize the whole in summer, and therefore they gain in 

 weight. It is remarkable that similar changes were observed 

 by IJr. Karl Theodor, Duke of Bavaria, in the amount of car- 

 bonic acid expired by a cat, in the case of which he measured 

 the expiration of this gas during five months. 



Many experiments have been made to find the combustion 

 heat of our food-stuffi. For want of direct animal calorimetrj', 

 physiologists used these data for calculating the heat produced 

 by living beings ; but as my experiments show, there is fre- 

 quently no exact accordance between the two. 



Richly nourished animals produce less, sparely nourished 

 ones more, heat than the calculation gives. Between the two 

 cases there is a third one in animals sufficiently nourished, viz. 

 such as take in so much nutriment as serves to maintain their 

 weight unchanged for a long time. In this case only the amount 

 of heat produced is really equal to that calculated upon the com- 

 bustion of the constituents of food. But also in this case 

 variations are observed, caused by change of temperature, 

 muscular motion or other circumstances, so that only the middle 

 figures correspond exactly to the theoretical value. 



Thus, if a well-nourished animal is starved the heat produc- 

 tion remains unchanged from three to four days, the animal 

 burning its stored-up materials and losing much of its weight ; 

 only then is it suddenly reduced to a lower amount. If now 

 food is given again, heat production remains small, the weight 

 increases, and then, three or four days later, the heat production 

 increases and reaches its former amount. 



If a sufficiently nourished animal takes in all its food once a 

 day, the heat production varies very regularly in the twenty- four 

 hours. Two hours after the meal it begins to rise, comes to its 

 maximum point between the fifth and seventh hour, falls sud- 

 denly between the eleventh and twelfth hour. In the second 

 half of the period the changes are small, the minimum poiut 

 being usually in the twenty-third hour. 



Similar changes go on in the expiration of carbonic acid. 

 But after the meal it rises much more rapidly, and therefore 

 comes earlier to its maximum point. Thus the ratio between 

 heat production and expiration of carbonic acid is not a 

 constant. This is true not only in the daily period. The 

 variations are seen to be still greater when we compare 

 different animals, or the same animal at diflferent times and in 

 different states of nutrition. 



By such researches we are enabled to examine more exactly 

 what chemical changes are going on in the animal system. The 

 materials afforded by food are all oxidized at last, and leave the 

 body in the form of carbonic acid and nitrogenous matter like 

 urea. What in a longer period is burnt in such a way, we can, 

 with a certain degree of exactness, make out by chemical ex- 

 amination of the constituents of food on the one hand, and of 

 the excretions on the other. We can make up, in such a way, a 

 balance account for gain and loss of the animal, like the balance 

 account of a merchant. But such an account gives no exact 

 i knowledge, because we have no means of completing it by taking 

 an inventory. We are, as regards the living body, in the 

 same position as a political economist, who knows the amount 

 of goods imported into and exported out of a country, but does 

 not know what has become of the goods stored up or used up 

 in the country itself. Therefore political economists do not 

 ' now regard the mere balance of trade as being so important 

 as they formerly thought. 



Physiology, like all branches of science, begins with a mere 

 description of processes observed. With the progress of our 

 knowledge, reason tries to connect these processes one with 

 another, and with those going on in lifeless nature. What we 

 call understanding is nothing else than knowing such connec- 

 tions. Now in the case of bodily income and expenditure, it 

 ! is easy to observe that all materials going out of the system are 

 more oxidized than those taken in as food, and reason tells 

 us that the combination of these food materials with the 

 ; oxygen inspired must be the source of animal heat. Hence, 

 we h.ave no doubt that the amount of heat produced must cor- 

 respond to the amount of chemical processes going on during 

 the same time. But these processes we cannot observe 

 directly ; we can only observe the final products car- 



NO. 1230, vol,. 48] 



bonic acids and others, when they leave the body. But by 

 some of the processes heat may be produced or absorbed with- 

 out any visible change of the body as a whole, viz. by solution 

 of solid matter, by splitting highly complex substances into 

 more simple ones, by forming sugar out of starch or glycogen 

 out of sugar. Considering this, we need not wonder that for 

 a long time it was impossible to answer the question whether 

 there is any other source of heat production in animals besides 

 oxidation. Only long continued calorimelric mrasurements' 

 have enabled me to fill up this gap.^ This done, I thought it 

 possible to discover also something about these inner processes, 

 by comparing, hour for hour, the heat production with the 

 excretion of carbonic acid, and with the absorption of oxygen. 



If the ratio between the heat produced and the carbonic acid 

 expired changes, this cannot be explained otherwise than by the 

 fact that different chemical substances are burned. Each sub- 

 stance, according to its chemical constitution, gives out, when oxi- 

 dized, a certain amount of carbonic acid, and produces a certain 

 amount of heat. But in the system it is a mixture of different 

 substances which come to be oxidized. This mixture changes, 

 not only in animals differently nourished, but also in the 

 same animal in different periods of digestion. After a rich 

 meal, what comes into the circulation first must be that part of 

 the food that is easily and rapidly digested and easily and 

 rapidly absorbed. Such substances are the proteid matters. 

 Later, the other constituents of the food, especially fat, come 

 to the tissues, where they are burned. Now/«/j, for the same 

 amount of carbonic acid, produce far more heat than proteids ; 

 so, during the first hours of digestion the afflux of oxidizablc 

 matter to the tissues being very great, both heat production 

 and expiration of carbonic acid increase, but the latter in a far 

 higher degree than the former. 



The animal body may be compared, as Prof. Huxley So 

 well says, to an eddy in a river, which may retain its shape for 

 an indefinite length of time, though no one particle of the water 

 remains in it for more than a brief period. But there is not 

 only the difference between the animal eddy and the eddy of 

 the river, viz. that the matter which flows into it has a different 

 chemical composition from the matter which flows out of it, 

 but in addition, matters which make up the eddy in a given 

 time, change, if I may so say, their chemical value, com- 

 bine with or separate from each other, without any visible change 

 of the whole system. 



The study of heat production is of the greatest value. No 

 doubt, the study of the vital processes becomes more complicated 

 when we take into account the invisible internal changes occurr- 

 ing in the body. But simplicity is not the highest aim in scientific 

 inquiries ; the highest possible exactness is that to which we must 

 aspire. Happily, the history of science shows that after trying 

 several ways to solve complex problems, we find that one of them 

 leads to a higher point of view, whence things appear in all 

 their completeness, simplicity and distinctness. Towards such 

 a point of view my researches are but the first step. Let us 

 hope that the united forces of many physiologists will shorten 

 the time necessary for the completion of the work. 



MAGNETIC PROPERTIES OF LIQUID 

 OXYGEN? 



A FTER alluding to the generous aid which he had received 

 ■^ both from the Koyal Institution and from others in con- 

 nection with his researches on the propeities of liquid oxygen, 

 and to the untiring assistance rendered hiui by his co-workers 

 in the laboratory. Prof. Dewar said that on the occasion 

 of the commemoration of the centenary of the birth of 

 Michael Faraday he had demonstrated some of the properties 

 of liquid oxygen. He hoped that evening to go several steps 

 further, and to show liquid air, and to render visible some of 

 its more extraordinary properties. 



The apparatus employed consisted of the gas-engine down 

 stairs, which was driving two compressors. The chamber con- 

 taining the oxygen to be liquefied was surrounded by two 

 circuits, one traversed by ethylene, the other by nitrous oxide. 

 .Some liquid ethylene was admitted to the chamber belonging to 

 its circuit, and there evaporated. It was then returned to the 



1 See also my address delivered to the general meeting of the German 

 Association of N.ituralisls at liremen, 1890. 



- Abstract of Friday evening discourse delivered at the Royal Institution 

 by Prof. Dewar, F.R.S. 



