Later experimenters in this field are Wroblewski, Olszewski, and Dewar, who have been interested mainly in the purely scientific side of the problem, while Linde in Germany, Hampson in England, and Tripler in the United States have their minds on the practical side. Notwithstanding the low temperatures involved in the experiments, a number of heated discussions have been carried on in the scientific journals touching the question of priority. To the unprejudiced observer it appears that all of those named above are entitled to credit. They have all helped the cause along, but just how to apportion the credit no one knows. In a general way, however, some of the results obtained by each in turn should be given. Wroblewski and Olszewski have carried on the work begun by Cailletet and Pictet, and have produced lower temperatures. In the latest form of apparatus used by Olszewski, liquid ethylene is used as the cooling agent. Its boiling point is -102° C. (-151.6° F.). By causing it to boil rapidly under diminished pressure a temperature below the critical temperature of oxygen can be reached. As early as 1891 Olszewski obtained as much as two hundred cubic centimetres of liquid air by this method. Dewar has also made use of liquid ethylene. This was passed through a spiral copper tube surrounded by solid carbonic acid and ether. It was then passed into a cylinder surrounded by another cylinder containing solid carbonic acid and ether. A spiral copper tube, which runs through the outer cylinder and also through the inner cylinder in which the ethylene was boiling under diminished pressure, carried the air. This was liquefied and then collected in a vacuum vessel below. Later he found that air can be liquefied by using liquid carbonic acid alone as the cooling agent. A sectional drawing of his apparatus described in 1896 is given herewith. As he remarks: "With this simple machine, one hundred cubic centimetres of liquid oxygen can readily be obtained, the cooling agent being carbon dioxide, at the temperature of -79°. If liquid air has to be made by this apparatus, then the carbonic acid must be kept under exhaustion of about one inch of mercury pressure, so as to begin with a temperature of -115°." [Illustration: FIG. 1.--LABORATORY LIQUEFACTION APPARATUS OF DEWAR FOR THE PRODUCTION OF LIQUID OXYGEN, ETC. A, air or oxygen inlet; B, carbon-dioxide inlet; C, carbon-dioxide valve; D, regenerator coils; F, air or oxygen expansion valve; G, vacuum vessel with liquid oxygen; H, carbon-dioxide and air outlet; o, air coil; ø, carbon-dioxide coil.]