That light had a deleterious effect upon micro-organisms was first discovered in this country by Messrs. Downes and Blunt, and their investigations led Professor Tyndall to carry out some experiments on the Alps, in which he showed that flasks containing nutritive solutions and infected with bacteria when exposed in the sunshine for twenty-four hours remained unaltered, whilst similar vessels kept in the shade became turbid, showing that in these the growth of bacteria had not been arrested. In these experiments mixtures of micro-organisms were employed, and the interest of the French investigations which followed lies in the use of particular microbes--notably the anthrax bacillus and its spores,[4] Roux demonstrating very conclusively that the bacillar form was far more sensitive to light than the spore form, while Momont, in a classical series of experiments, not only fully confirmed these observations, but showed also that the intensity of the action of light depends to a very large extent on the environment of the organism. Thus, if broth containing anthrax bacilli is placed in the sunshine, the latter are destroyed in from two to two and a half hours, whilst if blood containing these organisms is similarly exposed, their destruction is only effected after from twelve to fourteen hours of sunshine. This difference in resistance to insolation was also observed in the case of _dried_ blood and broth respectively--eight hours' exposure killing the bacilli in the former, whilst five hours sufficed in the latter. [4] In the interior of some bacilli there appears a round or oval body, having a very bright and shining lustre, which is known as a _spore_, and plays a most important part in the propagation of many kinds of bacilli. These spores are capable of resisting many hardships, which would be immediately fatal to the parent bacilli from which they have sprung. This is an instance of the apparent idiosyncrasies possessed by micro-organisms, which render their study at once so fascinating and so difficult, and it is through being thus constantly confronted with what, in our ignorance, we mentally designate as "whims," that we can hardly resist the impression of these tiny forms of life being endowed with individual powers of discernment and discrimination. Indeed, these powers of selection and judgment are in certain cases so delicately adjusted that in some of the modern chemical laboratories micro-organisms have become indispensable adjuncts, and by their means new substances have been prepared and fresh contributions made to the science of chemistry. Momont is not able to give any satisfactory explanation of this different behaviour of the anthrax bacilli in these two media, but goes on to show that yet another factor plays an important part during insolation.