Page:Popular Science Monthly Volume 25.djvu/78

70 We may discover, whenever the feet of the fly come again into contact with these tracks or minute drops, that they are composed of a very liquid substance, for they spread quite readily on the glass. We can not admit, as some naturalists assume, that the liquid can hold the club-shaped hair-ends by suction. If this were the case, the ends



would change shape during the suction, and would take the form of a disk. The fly puts its feet down and lifts them up with an incomparable facility that would not exist if the limb were really acted upon by the pressure of the air.

There is no evidence here of an adhesive substance; such a substance would harden after two or three days, and would dry or at least become viscous, like Venetian turpentine or sirup.

The power which we are investigating can be due only to capillary action; for the liquid and the hairs are the only parts that touch the polished surfaces. The idea occurred to me that the faculty arose from the attraction that each minute drop exercises upon the hair with which it is in contact; and I made several experiments to demonstrate the possibility of such an effect.

I suspended a hair from a pane of glass, by means of oil of olives. Sticking the cut end of the hair into the oil, I fixed the part, by means of the oil that adhered to it, to the glass, which I had previously washed with great care. I thus succeeded in suspending from the