THIS essay by Sir William Crookes, as will be remembered, called forth much criticism when it was delivered as the Presidential Address at the Bristol meeting of the British Association. Now that it is reprinted with additions, it may demand a more careful review than was possible at the time. The general idea, it may be said, is by no means novel, and it is one which approved itself to common sense. The population of the world and of different races in the world increases at rates which are more or less ascertainable. The means of supplying its wants, either in the shape of coal, or metals, or food, is also limited, either by the amount of the accumulated stocks, or the means of working them, or the extent of the earth's surface available for producing the food. What more natural then than to calculate, as has been done by Malthus, as to food generally, by other statisticians as to food in particular districts, such as India at the present time, by Jevons as to coal in England, by Prof. Cairnes as to metals generally, and by Prof. Suess, an Austrian geologist and statistician, as to gold specially, that at a given date in future the supply must run short, and then a crisis arrive? Sir William Crookes applies specially to the study of wheat production this leading idea. The consumers of wheat, he tells us, are a little over 500 millions at the present time, having increased to that figure from about 370 millions in 1870, while the quantity of the earth's surface available for wheat is itself limited both by the capacity of the soil and the necessity for cultivating other products. He calculates accordingly that by the year 1931, if population increases as it has done, the supply of wheat will be seriously short, and he suggests that chemistry should come to the rescue by devising means to fix the nitrogen of the atmosphere so as to permit the growth of more wheat on the same soil. The Wheat Problem. Based on remarks made in the Presidential Address to the British Association at Bristol in 1898. Revised, with an answer to various critics. By Sir William Crookes. (London: John Murray, 1899.)