24 Mr. J. Brown on 



announced a similar achievement. Following oxygen, all the 

 other gases known at that period soon yielded likewise. About 

 the case of hydrogen it is true doubts and difficulties seem to 

 hang. At all events six years elapsed before Wroblewski and 

 Olszewski obtained hydrogen in the form of a static liquid, 

 and to collect liquid hydrogen in some quantity, as Professor 

 Dewar remarks, has taken twenty years from the date of 

 Pictet's experiments. 



Though Siemens had suggested the principle earlier, it was 

 about 1895 that Linde, and also Hampson, devised perfect and 

 simple apparatus. It is true that others in the meantime were 

 approaching perfection, Dewar, for instance, having described 

 in 1886 an apparatus embodying this principle. The chief 

 difficulty is the production of a sufficient degree of cold. 

 Lowering of temperature is, in modern apparatus, attained by a 

 device which I shall try to explain in a simple way. In the 

 antique apparatus for obtaining fire known as the fire syringe 

 (a specimen of which has been kindly lent us by Mr. Robert 

 May, whose interesting collection of antique candlesticks is at 

 present on view in the Free Library) heat is obtained by the 

 compression of air. The syringe consists of a strong brass 

 tube with an airtight plunger reaching nearly to the bottom of 

 the tube. On driving the plunger rapidly down, the air 

 beneath is so heated as to set fire to a piece of touch cotton 

 (cotton wool treated with saltpetre and sulphur or with solution 

 of phosphorous), which has previously been attached to the 

 end of the plunger. When the plunger is withdrawn the air, 

 which has been hot enough to set fire to our cotton wool, cools 

 again. To do so it absorbs heat. From this simple experiment 

 we conclude generally that air in expanding cools itself. Thus 

 by the device of first compressing air, and, as it were, squeezing 

 some of its heat out, and then allowing it to expand again, we 

 cool it below the temperature at which we started. 



This is the first process in the air liquefying apparatus. Air 

 is compressed in water-cooled pumps to 120 atmospheres, say 

 i,7oolb. per square inch, further cooled by passing through 



