POPULAR SCIENCE MONTHLY.
and his associates feel justified in the following conclusion: "The mosquito serves as the intermediate host for the parasite of yellow fever, and it is highly probable that the disease is only propagated through the bite of this insect."
One of the most obscure points in chemistry is the action of ferments. These have been grouped in two classes: Organized ferments like the yeast plant, or the mycoderma aceti, which oxidize alcohol to acetic acid; and the unorganized ferments, like diastase, which convert starch into sugar. In both cases a very small quantity of the ferment is capable of converting an indefinitely large amount of the fermenting substance into the fermented product, although the ferment itself does not enter as such into the reaction. Further, the action of ferments can be inhibited by heat and by the action of certain substances which act as poisons. Recent investigations seem to show that the organized ferments may owe their action to unorganized ferments which they secrete. More recently attention has been called by Bredig and von Berneck to the similarity between the action of ferments, and what has been called contact action of metals. For example, finely divided platinum can oxidize alcohol to acetic acid, and can invert cane sugar. Much more marked is the action of a solution of colloidal platinum, obtained by passing a strong current of electricity between platinum poles under water. The action of the platinum in this condition is remarkably like that of a ferment. When its effect upon hydrogen peroxide was studied it was found that one part in about 350,000,000 parts of water was sufficient to decompose hydrogen peroxide appreciably. Minute traces of certain poisons affect the reaction strongly; especially is this true of prussic acid, hydrogen sulfid and corrosive sublimate. Like many ferments the platinum solution gradually recovers from the poisonous effects of traces of potassium cyanid. It also appears that the platinum plays no chemical part in the reaction, and thus it is apparently a true ferment. It seems probable that the study of these inorganic ferments may throw much light upon the action of the very complicated organic ferments.
When the discovery was made some ten years ago that leguminous plants are able to assimilate the free nitrogen of the atmosphere, and thus to supply themselves with one of the necessary elements of plant food, its importance to agriculture as an economical means of maintaining soil fertility was recognized almost immediately. In working out the practical application of the discovery it was found that the micro-organisms which effect this nitrogen assimilation are not the same for all kinds of legumes, but that different kinds have their specific organisms, and furthermore that these micro-organisms are not universally disseminated through the soil. This led to inoculation of the soil, either with pure cultures of the specific bacteria or with soil from a field known to contain them in abundance. What seemed so simple theoretically has been found in practice to be only partially successful, so that the progress in its application has been somewhat delayed. A very interesting account of experiments in inoculating soils for the growth of the soy bean has recently been published by the Kansas Experiment Station as Bulletin No. 96. It is one of the most successful attempts at soil inoculation on a large scale that has been reported in this country or in Europe, where this method for promoting nitrogen assimilation was first suggested. It-was found that the Kansas soil contained none of the organisms necessary for the soy bean, and that in such soil the roots produced none of the tubercles which are intimately associated with nitrogen assimilation. A quantity of soil was obtained from the Massachusetts Experiment Station, where the soy bean had been grown for several years, and mixed in very small proportion with the Kansas soil, with the result that