3^ 



NATURE 



[November 12, 1891 



steam-engines, is intended to withdraw heat from the 

 objects under observation, and to keep them at any tem- 

 perature between - 20° and - 200° C. as long as may be 

 required. Prof. Pictet's experience has led him to the 

 conclusion that among the refrigerating agents known, 

 such as rarefaction of gases, dissolution of salts, evapora- 

 tion of liquids, the latter is to be preferred. A long 

 course of research has further enabled him to choose the 

 most convenient from amongst the great number of suit- 

 able liquids. In order to avoid the great pressure re- 

 quired in handling the highly evaporative substances of 

 lowest boiling-point which serve to produce extreme 

 cold, it is necessary to divide the difference of tempera- 

 ture into several stages. Each stage is fitted with 

 especial apparatus consisting of an air-pump worked by 

 steam, which drains off the vapours of the liquid from the 

 refrigerator, and forces them into a condenser, whence, 

 reduced to the liquid state, they are again offered for 

 evaporation in the refrigerator. Thus the liquid, without 

 any loss beyond leakage, passes through a continuous 

 circuit, and the operations can be carried on for any 

 length of time. The liquid made use of for the first 

 stage is the mixture of sulphurous acid and a small 

 percentage of carbonic acid called " liqidde Pictet." It is 

 condensed at a pressure of about two atmospheres in 

 a spiral tube merely cooled by running water. Oxide of 

 nitrogen (laughing gas) is the liquid chosen for the 

 second stage. Its vapours are condensed in the 

 same way at a pressure about five or six times as great 

 in a tube maintained at about — 80° by the action of the 

 first circuit. As medium for a third stage, in which, 

 however, continuous circulation has not yet been at- 

 tempted, atmospheric air is employed, which passes into 

 the liquid state at a pressure of no more than about 

 75 atmospheres, provided the condenser is kept at - 135^ 

 by the first two circuits. The evaporation of the lique- 

 fied air causes the thermometer to fall below - 200°. 



By this combination quite new conditions for investi- 

 gating the properties of matter are realized. In various 

 branches of science new and surprising facts have already 

 been brought to light. Many laws and observations will 

 have to be re-examined and altered with regard to changes 

 at an extremely low temperature. 



For instance, a remarkable difference was noted in the 

 radiation of heat. Material considered a non-conductor 

 of heat does not appear to affect much the passage of 

 heat into a body cooled down to below — 100°. Or, to state 

 the fact according to Prof. Pictet's view: "The slow 

 oscillations of matter, which constitute the lowest degrees 

 of heat, pass more readily through the obstruction of a 

 so-called non-conductor than those corresponding to a 

 higher temperature, just as the less intense undulations 

 of the red light are better able to penetrate clouds of dust 

 or vapour than those of the blue." If the natural rise of 

 temperature in the refrigerator, starting from -135°, is 

 noted in a tracing, and afterwards the same refrigerator 

 carefully packed in a covering of cotton-wool of more 

 than half a yard in thickness, and cooled down afresh, 

 and the rise of temperature again marked, on comparing 

 the tracings hardly any difference will be found in the 

 two curves up to - 100") and only a very slight deviation 

 even up to - 50°. On this ground it is clear that the utmost 

 limit of cold that can possibly be attained is not much 

 lower than that reached in the famous experiment of 

 liquefaction of hydrogen. The quantity of warmth which 

 hourly floods a cylinder 1250 mm. high by 210 mm. wide 

 (the size of the refrigerator) at — 80"", is no less than 600 

 calories, and no packing will keep it out. At a lower 

 temperature, the radiation being even greater, the power 

 of the machinery intended to draw off still more heat 

 would have to be enormous. And as —273'^ is absolute 

 zero, the utmost Prof. Pictet judges to be attainable is 

 about - 255". 



As an example of the surprising methods which the 



NO. I 1 50, VOL. 45] 



refrigerating machine permits the investigator to employ, 

 it may be mentioned that, in order to measure the 

 elasticity of mercury. Prof Paalzow had the metal cast 

 into the shape of a tuning-fork, and frozen hard enough 

 for the purpose in view. On this occasion it appeared 

 that quicksilver can be shown in a crystallized state, the 

 crystals being of a beautiful fern-like appearance. 



Glycerine was likewise made to crystallize ; and cognac, 

 after having been frozen, was found to possess that peculiar 

 mellowness commonly only attained by long keeping. 



But the most important application of the refrigerating 

 machinery has been the purification of chloroform, under- 

 taken by Prof. Pictet at the instance of Prof Liebreich, 

 of the Pharmacological Institute, Berlin. Chloroform 

 has hitherto been considered a most unstable and easily 

 defiled substance. The action of sunlight, the slight 

 impurities retained from the different processes of manu- 

 facture, perhaps the mere settling down during protracted 

 storage, have invariably resulted in a more or less marked 

 decomposition. By the simple process of crystallization 

 this unstableness is got rid of, and a practically un- 

 changeable liquid is produced. The crystals begin to 

 form at - 68°, first covering the bottom of the vessel, and 

 gradually filling it up to within one-fifth of the whole 

 volume. This residue being drained off, the frozen part 

 is allowed to melt under cover, so as to exclude the 

 atmospheric moisture. Chloroform thus refined has, by 

 way of testing its durability, remained exposed on the 

 roof in a light brown bottle from November till June 

 without the slightest sign of decomposition. 



Prof Pictet has already taken steps to introduce his 

 process into manufacture, and proposes to apply the 

 principle to various other chemical and technical objects. 

 Sulphurous ether, for instance, has by a similar process 

 been .produced in a hitherto unknown degree of purity. 

 At the same time, the Professor continues eagerly to 

 pursue the various purely scientific inquiries with which 

 he started. R. DU Bois-Revmond. 



RESULTS OF EXPERIMENTS AT ROTH- 

 AM S TED ON THE QUESTION OF THE 

 FIXATION OF FREE NITROGEN} 

 Tip ROM the results of the experiments of Boussingault, 

 -*■ and also of those made at Rothamsted under con- 

 ditions of sterilization and inclosure more than thirty 

 years ago. Sir J. B. Lawes and the author had always 

 concluded that at any rate our agricultural plants did not 

 assimilate free nitrogen. They had also abundant evidence 

 that the Papilionaceae, as well as other plants, derived 

 much nitrogen from the combined nitrogen in the soil 

 and sub-soil. Still, they had long recognized that the 

 source of the whole of the nitrogen of the PapilionaceEe 

 was not explained ; that there was, in fact, " a tnissing 

 link " ! They were, therefore, prepared to recognize the 

 importance of the results first announced by Prof. Hell- 

 riegel in 1886 ; and they had hoped to commence experi- 

 ments on the subject in 1887, but they had not been able 

 to do so until 1888. Those first results showed a con- 

 siderable formation of nodules on the roots, and coin- 

 cidently great gain of nitrogen, in plants grown in sand 

 (with the plant-ash) when it was microbe-seeded by a 

 turbid watery extract of a rich soil. 



In 1889 and since, they had made a more extended 

 series. The plants were grown in pots in a glass-house. 

 There were four pots of each description of plant, one 

 with sterilized sand and the plant-ash, two with the same 

 sand and ash, but microbe-seeded with watery extract, 

 for some plants from a rich garden soil, for lupins from a 

 sandy soil in which lupins were growing luxuriantly, and 



' Abstract of a paper read before the Agricultural Chemistry Section of 

 the Naturforscher Versaramlung at Halle a.S., by Dr. J. H. Gilbert, 

 F.R.S., September 24, 1891. 



