Two lectures have been recently delivered by Prof. James Dewar at the Royal Institution on the subject of liquid and solid hydrogen. These lectures have been illustrated by experiments and have attracted the attention of the most distinguished chemists and physicists of England. It is easy to understand such interest in the subject when we consider that even Clerk Maxwell thought it improbable that hydrogen would ever be liquified, and yet Dewar was able to exhibit not only liquid, but solid, hydrogen to his audience. Briefly recapitulated, the steps in the condensation of what were formerly called the permanent gases are these: in 1878 Cailletet, in Paris, and Pictet, at Geneva, by suddenly expanding gases which had been compressed to a high degree and cooled to a low temperature, succeeded in obtaining these gases in the shape of a mist or of a transitory liquid jet. In 1884 Wroblewski and Olszewski at Crakow obtained oxygen and nitrogen as static liquids. By expanding hydrogen from a compression of 190 atmospheres in a vessel cooled by liquid air evaporating under diminished pressure, this gas was obtained as a mist or momentary froth, though it was affirmed by Olszewski that he observed the liquid hydrogen in colorless drops and as a liquid running down the sides of the tube. In May, 1898, Dewar obtained hydrogen as a static liquid by allowing compressed hydrogen, cooled in a bath of boiling air, to escape rapidly at a jet, the liquid hydrogen being collected in a doubly isolated vacuum vessel. This liquid hydrogen is a colorless liquid, with a specific gravity of 0.07 or less than one sixth the weight of liquid marsh gas, the lightest liquid hitherto known. This is better realized by saying that while one gram of water has the volume of one cubic centimeter, one gram of liquid hydrogen has a volume of over 14 c. c. The boiling point of hydrogen is -252° C. or 21° above the absolute zero, and by boiling in a vacuum the temperature of 15° can be obtained. Very recently by slowly evaporating very perfectly isolated liquid hydrogen, solid hydrogen was obtained by Dewar as a white mass of solidified form, of the lowest temperature ever obtained, -258° C. * * * * *