Page:Popular Science Monthly Volume 25.djvu/76

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known to the laundress, and to all who have seen cooked arrowroot. If this paste be dried by evaporation, it does not regain its former insolubility, but readily dissolves in hot or cold water. This is what I should describe as cooked starch.

Starch may be roasted as well as boiled, but with very different effects. The changes that then occur are much more decided, and very interesting. I will describe them in my next.—Knowledge.


IT was believed at one time that flies and some other insects owe the faculty of running over smooth bodies like glass to the numerous hairs with which their feet are provided catching in the pores of the material. The absurdity of this supposition is readily apparent on examining glass with the microscope; and no naturalist can be found in these days to uphold it. Another theory, which has been frequently advanced, explains the fact by affirming that the feet terminate in little suckers, by the application of which to the smooth surface the insect is able to adhere by the force of the pressure of the air, in the same manner that the street-boy fastens his leather sucker tightly to the flagging. Blackwall's investigations have demonstrated that such a contact as is here supposed does not take place. He has seen flies running over the inner sides of the bell-glass receiver of an air-pump from which the air had been exhausted. If we examine the foot of a fly through the microscope, we shall find that there are no suckers on it, but that the foot-cushions are furnished with very fine hairs that prevent all close contact with the glass. The theory in question which invokes the pressure of the air was first broached by Dr. Derham, and was accepted by most of his contemporary entomologists. Other observers, among them Dr. Hooke, were of the opinion that the insects were able to attach themselves to the glass by virtue of some sticky matter in or on the hair. Blackwall explained the fact by saying that a viscous substance flowed from each hair; and probably the majority of the later entomologists have accepted this explanation. In answer to it, we may say that, if there really were a flow of a viscous fluid from the hairs, the flies would not be able to move after they had rested in one spot for a little while, for the liquid would have dried or hardened so as to detain them; but we know that the insect can always fly away instantaneously, even if it has remained in the same place for hours without moving.

I have concluded from my experiments that it is not the pressure of the air nor the power of an adhesive liquid that gives flies the fac-