April 2, 1890J 



NA TURE 



515 



other, and the market will nevertheless be regularly 

 supplied. 



What, then, is a working plan ? The German term is 

 Wirthschafts plan, and the English term (working plan) 

 was first used in 1856, when the writer of these lines 

 commenced to work the Pegu teak forests on a regular 

 system. The number of teak trees of the different age 

 classes was approximately determined by an elaborate 

 system of valuation surveys. It was found that the trees 

 of the second class were sufficiently numerous to take the 

 place of the first class trees, and that the same was the j 

 case with the younger classes. It was also ascertained, 

 that twenty-four years on an average would be required 

 for the trees of the second class to attain first class size. 

 The result was that the removal of the first class trees, 

 those which were fit to yield marketable timber, must be 

 spread over at least twenty-four years ; and upon this very 

 simple principle, a working plan, intended to provide, in 

 the first instance, for six years only, was established for 

 the different forest districts. After the expiration of the 

 first six years, this plan was renewed, and subsequently 

 modified and elaborated in detail. The principle, how- 

 ever, has been maintained to the present day. These 

 are the bare outlines of the scheme, which has not only 

 ensured a sustained yield, but, and that is very im- 

 portant, has been readily intelligible to all. 

 {Tq be cQnHmted.) 



tion was given, at Brin's Oxygen Works, of the construc- 

 tion and use of a new apparatus, the subject of an 

 English patent, dated May 23, 1895, standing in the 

 name of Dr. William Hampson. The apparatus consists 

 of three coils of narrow copper tubing, arranged con- 

 centrically in a metal case, and connected successively 

 together, as shown in the accompanying diagram (Fig. i), 

 which displays a vertical section of the apparatus. The 

 gas, say oxygen, enters the outer coil under a pressure 

 of 120 atmospheres, passing from this into the second, and 

 from this into the central coil, which is surrounded by a 



THE NEW PROCESS FOR THE LIQUEFAC- 

 TION OF AIR AND OTHER GASES. 

 'X*HE liquefaction of air, and the rest of the so-called 

 -*• permanent gases, is an achievement which belongs 

 to quite recent times. Faraday cooled and compressed 

 gases by such means as were at his disposal, with results 

 which are well known ; but it was the experiments of 

 Andrews, published in 1869, which taught physicists the 

 fact that until the cooling has been effectual no amount 

 of pressure will liquefy the gas ; in fact, that every gas 

 has a critical point below which its temperature must be 

 reduced before pressure can bring about liquefaction. 

 The critical points of oxygen and the components of air 

 are very low. Hence it was not till 1877 that these gases 

 were liquefied by Pictet and by Cailletet. The former 

 reached the necessary temperature by two stages, using 

 first liquid sulphur dioxide, then liquid carbon dioxide, 

 both boiling under reduced pressure. Cailletet used 

 the principle of cooling by sudden release from 

 higher to lower pressure. The introduction of liquid 

 ethylene as a cooling agent enabled experimenters to 

 make another step forward ; for, with the help of 

 liquid ethylene, Wroblewski and Olszewski first obtained 

 liquid oxygen in quantity far larger than would be possible 

 in any form of Cailletet's apparatus, and without the 

 complicated machinery of Pictet. Liquid oxygen itself 

 thus became available as a refrigerating agent, and 

 afforded the means of cooling a tube containing any 

 other gas to a temperature lower than ever ; namely, 

 about 211' below zero Centigrade. With this cooling 

 agent, and with the further cooling produced by expan- 

 sion of the confined gas from a pressure of 150 atmo- 

 spheres to 20 atmospheres, hydrogen has been liquefied 

 by Olszewski. Suggestions have from time to time been 

 made as to the possibility of applying the reduction of 

 temperature, consequent upon the expansion of a gas 

 when released from a high pressure, to the further cool- 

 ing of the compressed gas ; but no practical steps had 

 been taken in this direction till the publication, in October 

 last, of Herr Linde's successful liquefaction of air by the 

 application of this principle. It now appears, however, that 

 Linde has not only been anticipated in the application of 

 the principle, but that a more effective apparatus than his 

 has been devised. On Saturdav, March 21, a demonstra- 



NO. 



1379, VOL. 53] 



-Sectional elevatii 



-Detail of valve. 



cylindrical glass vacuum-jacketed vessel as devised by 

 Prof. Dewar. The two outer coils are separated from 

 each other by vertical divisions of the case, and the 

 spiral of the central coil is followed by a flat spiral of 

 sheet copper. When the gas reaches the extremity of 

 the central coil, it escapes through a fine orifice of peculiar 

 construction, formed by bringing two knife-edges closely 

 together (shown in Fig. 2). The size of the orifice can be 

 regulated by means of an ebonite rod, which passes up 

 the axis of the apparatus, and terminates in a handle at the 

 top. After its escape the whole of the gas cooled by 



