mixtures; these solutions seem to show an expansion when the amount of benzene is equal to or greater than the amount of alcohol, but when the alcohol is greatly in excess a small contraction is observed. The extent to which the theory dealing with this phenomenon is presented will depend upon the previous training of the students concerned. In elementary classes it will probably suffice to call attention to the fact that a positive heat of solution or of mixing is usually accompanied by a contraction in volume; while a negative heat of solution accompanies an expansion of volume. In an ideal solution (one which obeys Raoult's law) there is no heat of solution and no volume change. The nearer a solution is to being an ideal solution the smaller will be the volume change and the less will be the heat of solution. J. H. Hildebrand' gives a more nearly complete discussion of the above relationships and several references to the literature. '"Solubility," J. H. Hildebrand, A. C. S. Monograph, Chemical Catalog C o , New York, 59-G8 (1924).
Cause of Cancer Unlmown Despite Many Theories. I n spite of much research and many theories nothing is yet known of the cause of cancer, if i t has a single cause, Dr. Shields Warren, of the Palmer Memorial Hospital, told his audience a t a recent Harvard Medical School public lecture. "While there is no one accepted cause of cancer, there are certain theories that are useful as working hypotheses," said Dr. Warren. The reported discoveries, from time to time, of a parasite as a cause of cancer have raised hope. Among these Dr. Warren mentioned the work of Dr. Gye in England which unfortunately was not confirmed. Many theories that special foods or substances cause cancer bave been raised. Civilization has been blamed as a cause of this disease. But all of these theories bave proved untenable. The theory of chronic irritation covers many of the cancers that occur in human beings, but it is t o a certain extent a supetdcial explanation, Dr. Warren declared. Prolonged chronic irritation of tissues stimulates cell growth and brings about unknown changes in the tissues which favor the development of cancer. Many types of chronic irritation do not go on to develop cancer, but many cares of cancer may be traced to long standing irritation. Heredity has been considered a cause of cancer. Proof of this theory has ranged from examples of xrcalled cancer families or cancer villages to the painstaking work of Dr. Maude Slye. Dr. Slye proved by very careful and extensive experiments t h a t in mice there is a hereditary predisposition to cancer, but it is questionable as t o whether her findings are applicable t o human beings, Dr. Warren explained. The so-called cancer villages are usually found to be inhabited largely by older people among whom the prevalence of cancer is always greater, no matter where they live. "Certainly we are safe in saying that a t the present time heredity is not considered of importance as a cause of cancer," Dr. Warren declared. While the cause of cancer is still unknown, enough facts are known about cancer to make its behavior less incomprehensible and its treatment more hopeful."-Science Service
