Page:Popular Science Monthly Volume 53.djvu/735

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is written in English, and the second letter from America is written in French. The correspondence thus commenced is continued as begun, alternating the two languages; also all letters received which are written in the language of the receiver are returned carefully corrected to the writer. Thus, if letters are filed, at the end of the year each student has model letters in the foreign tongue, and his own corrected letters for careful study. In Swarthmore College, Pennsylvania, about thirty students are enrolled as correspondents. The letters as received are read aloud in class, sometimes translated, sometimes in French, and are made most interesting topics for the lesson of the day. No one who has not tried the system can fully realize the new life and spirit that are thus infused into the class. Instead of being a dry and dull grammatical lesson, with little direct practical bearing upon daily life, the language is seen at once to have a life and meaning before little expected by the student."

The Liquefaction of Hydrogen.—At a recent meeting of the Royal Society, Professor Dewar read a paper describing the method by which on Tuesday, May 10th, he had succeeded in liquefying hydrogen, the last of the so-called "permanent gases." The apparatus used was a year in building, many slight but important details going to make up the final successful machine. The hydrogen was cooled to —205° C. and then allowed to escape continuously under a pressure of one hundred and eighty atmospheres from the nozzle of a coil of pipe at the rate of ten to fifteen cubic feet a minute, into a vacuum vessel doubly silvered, which was itself surrounded with a space kept below  —200° C. With these arrangements liquid hydrogen began to drop from this vessel into a second vacuum space, doubly isolated by being inclosed in a third, and in five minutes twenty cubic centimetres of liquid hydrogen were collected. The yield of liquid was about one per cent of the gas. The liquid was clear and colorless and showed no absorption spectrum. When a long piece of glass tubing sealed at one end and open to the air at the other was immersed in the liquid hydrogen, solid air immediately appeared in it; and when a specimen of purified helium in a sealed tube was immersed, a distinct fluid was seen to collect in it. It would seem from this that there is little difference between the boiling points of helium and hydrogen. Professor Dewar pointed out that all known gases had now been condensed into liquids which could be manipulated at their boiling points under atmospheric pressure in suitably arranged vacuum vessels, though even so great a man as Clerk Maxwell had doubts as to the possibility of ever liquefying hydrogen. With liquid hydrogen as the cooling agent a temperature could be reached within 20° or 30° of the zero of absolute temperature, and its use would open up an entirely new field of scientific inquiry. M. Moissan read a similar paper before the Academy of Science in Paris early in May. It is also claimed that Professor Olszewski had previously determined the boiling point and critical temperature of hydrogen.

Grasses in Iowa, Nebraska, and Colorado.—Prof. L. H. Pammel, in his Notes on the Grasses and Forage Plants of Iowa, Nebraska, and Colorado, remarks on the different aspects the forage question in central Iowa presents now from what it did fifteen years ago. At that time considerable areas of unbroken sod still remained. Now the wild prairies have ceased to be a factor in the production of hay. They have given way to cultivated fields and pastures, and the few small unbroken areas occurring here and there are chiefly confined to the small drainage basins between the hills where moisture interferes with proper cultivation. The standard and other cultivated grasses that have been introduced have been tried with varying success; the native species vary in quantity and quality in different parts of the State. Several native leguminous plants have more or less value for fodder. The pastures suffer deterioration from overstocking and the growth of weeds. The grasses can not endure the close grazing and extensive trampling to which they are subjected, and die out, and weedy annuals plant themselves in their places, or the native ragweeds and verbenas spread and occupy the soil. All of these have become so plentiful that farmers note their more frequent occurrence than in former years. In Nebraska the grasses do not grow so luxuriantly season