184 MAGNETIC PROPERTIES OF LIQUID OXYGEN. 



about oiie-sixtli that of water. The temperature of liquid oxygen at 

 atmospheric pressure, determined by tlie specific heat method, using 

 platinum and silver, was —180° C. 



Eeferonce was then n^ade to a remarkable exf)erimental corroboration 

 of the correctness for exceedingly low temperatures of Lord Kelvin and 

 Prof. Tait's thermoelectric diagram. If the lines of copper and i)lati- 

 num were prolonged in the direction of negative temperature, tliey 

 would intersect at — 95o C. Similarly, the copper and palladium lines 

 would cut one another at —170° C. Now, if this diagrm were correct, 

 the E. M. F. of the thermo-electric junctions of these two pairs of metals 

 should reverse at these points. A Cu — Pt junction connected to a 

 reflecting galvanometer was then placed in oxygen vapor and cooled 

 down. At —100° C. the spot of light sto])ped and reversed. A Cu — Pd 

 junction was afterwards placed in a tube containing liquid oxygen, and 

 a similar reversal took place at about —170° 0. 



Liquid oxygen is a non-conductor of electricity; a spark taken from 

 an induction coil, one millimeter lojig in the liquid, requires a potential 

 equal to a striking distance in air of 25 millimeters. It gave a flash 

 now and then, when a bubble of the oxygen vapor in the boiling liquid 

 came between the terminals. Thus liquid oxygen is a high insulator. 

 When the spark is taken from a Wimsliurst machine the oxygen appears 

 to allow the passage of a discharge to take place with much greater 

 ease. The spectrum of the spark taken in the liquid is a continuous 

 one, showing all the absorption bands. 



As to its absorption spectrum, the lines A and B of the solar spectrum 

 are due to oxj^gen, and they came out strongly when the liquid was 

 interposed in the path of the rays from tlie electric lamp. Both the 

 liquid and the highly compressed gas show a series of five absorption 

 bands, situated resi)ectively in the orange, yellow, green, and blue of 

 the spectrum. 



Experiments prove that gaseous and liquid oxygen have substantially 

 the same absori^tion spectra. This is a very noteworthy conclusion, 

 considering that no compound of oxygen, so far as is known, gives the 

 absorptions of oxygen. The persistency of the absorption through the 

 stages of gaseous condensation towards complete liquidity implies a 

 persistency of molecular constitution which we should hardly have 

 expected. The absorptions of the class to which A and B belong must 

 be those most easily assumed by the diatomic molecules (Go) of ordinary 

 oxygen; whereas the diffuse bands above referred to, seeing they have 

 intensities proportioned to the square of the density of the gas, must 

 depend on a change produced by compression. This may be brought 

 about in two ways, either by the formation of more complex molecules, 

 or by the constraint to which the molecules are subjected during their 

 encounters with one another. 



When the evaporation of li(iuid oxygen is accelerated by the action 

 of a high expansion pump and an open test-tube is inserted into it, the 



