ON DRY, POLISHED, VERTICAL SURFACES. 229 
possibly be were it of the tenacity implied by the 
foregoing conjecture. 
That flies are unable to walk on polished vertical 
surfaces when breathed upon till the aqueous vapour 
expelled from the lungs is copiously condensed thereon 
is an acknowledged fact ; but it does not appear to 
be known that when thus treated they cannot even 
retain the position they occupy, whether they make 
any visible effort to do so or not, a circumstance that 
seems to be quite inexplicable on the hypothesis that 
they are supported by the agency of atmospheric 
pressure, but which admits of a satisfactory expla- 
nation on the principle of a solvent fluid acting 
upon a gelatinous and moderately adhesive animal 
secretion; and these remarks apply to numerous 
species of insects, and also to spiders provided with 
scopule ; but the latter, when they perceive their 
footing to be insecure, frequently attach themselves 
to the spot by emitting from their spinners a little 
of the viscid material of which their silken lines are 
formed, that possesses the property of being insoluble 
in water. 
In spring, summer, and autumn House-flies may 
frequently be seen adhering so firmly to the upright 
surface of the glass of windows that they are incapable 
of extricating themselves though they make every ex- 
ertion to accomplish that object ; yet, when breathed 
upon till the aqueous vapour exhaled is condensed 
about them, they speedily fall from the spot to which 
