248 Annals of the Philosophical Club 



June 2oth, 433rd meeting. Dr. Sclater exhibited speci- 

 mens of the silicified wood (Nicolia), which he had brought 

 from the ' petrified forest ' in the Suez desert, some 7 miles 

 from Cairo in a direction south of east. From the way 

 the trunks were lying he had little doubt the trees had 

 been brought down by water and stranded on the shore 

 of an ancient estuary. 



Nov. 21 st, 435th meeting. In the absence of Professor 

 Dewar, Mr. Crookes described a simple, rapid, and inexpen- 

 sive method of preparing liquid air. One end of a strong 

 copper tube, about a quarter of an inch in external diameter, 

 is connected with a steel bottle, about one cubic foot in 

 capacity, containing air at 180 atmospheres pressure. The 

 other end (a few feet distant) is twisted into a close spiral 

 about nine inches long and two inches in external diameter. 

 Inside this a glass tube, exhausted to a high vacuum, is 

 tightly slipped, and the whole put into a vacuum- jacketed 

 glass cylinder a couple of inches longer than the copper-spiral, 

 the end of which is open. On turning the tap of the high- 

 pressure cylinder, the compressed air rushes into the lower 

 part of the outer cylinder. In expanding it abstracts heat 

 from surrounding bodies, but as the vacuum jacket, inside 

 and out, almost completely isolates thermally the upper 

 spiral, the temperature of the stream of expanding air is 

 lowered till it reaches its own liquefying point, when liquid 

 air begins to roll down the outer cylinder. Last Saturday 

 he saw Professor Dewar perform the experiment at the 

 Royal Institution, when liquid air began to appear in about 

 five minutes after the tap was turned on, and in ten minutes 

 seventy cubic centimetres of liquid air had collected, the 

 compressed air expended amounting to about fifty cubic 

 feet at the normal atmospheric pressure. If the high 

 pressure cylinder be filled with oxygen or nitrogen, they also 

 are liquefied, and with a similar apparatus Professor Dewar 

 has succeeded in obtaining liquid hydrogen. 1 



1 A fuller account of the experiments, with figures of the apparatus, is given 

 in Nature, vol. liii. pages 329-331, from a paper read by Professor Dewar to- 

 the Chemical Society on Dec. iQth, 1895, and printed in their Proceedings. 



