192 Prof. G. Luvini's Experiments and Observations 



ing of adhesion, I wrote in my Treatise the following; paragraph 

 on purity of surface : — "If you wish to study the adhesion between 

 two bodies, it is of great importance that the surfaces of contact 

 be clean and free from every other substance, such as moisture, 

 grease, dust, &c. Indeed if there be any extraneous substance 

 between two bodies, it will adhere to them with greater or less 

 energy, depending on the magnitude and continuity of the stra- 

 tum, and so modify the effect in question. Absolute purity of 

 the surfaces is all but impossible to obtain, since the air itself, or 

 the substances used in cleansing, will adhere in some way or 

 other to the surfaces already partially clean, and will deposit on 

 them dust or extraneous matters, which all concur to modify the 

 force of adhesion between the surfaces." 



On first reading the account of M. Plateau's experiments, it 

 occurred to me that the superficial stratum of liquids would so 

 far differ from the lower strata inasmuch as the former is exposed 

 to the dust of the air. In order to test this I repeated some of 

 M. Plateau's experiments in the air, and so far reproduced his 

 results as to satisfy myself that I was acquainted with his mani- 

 pulation. I then put the needle into water contained in a wide 

 glass receiver, and turned over this another receiver full of water 

 just as if I were going to collect a gas generated by the needle 

 itself. This second receiver, which was narrower than the first, 

 was lowered to the bottom of the first so as completely to shut in 

 the needle. The receiver being properly supported, I began a 

 series of observations in order to determine the time occupied by 

 the submerged needle in traversing 85°. I then gently intro- 

 duced gas into the inverted receiver until the water was so far 

 displaced that the lower surface of the needle rested on the water 

 while the upper was in contact with the gas. I then once more 

 determined the time occupied by the needle in traversing the 

 85°. The gases used in these observations were air, hydrogen, 

 carbonic acid, and oxygen. The results in these different gases 

 were approximate, at least within the probable errors of expe- 

 riment. 



I must, however, admit that this method of experimenting is 

 somewhat uncertain and troublesome. If the level of the liquid 

 descend too low, one end of the needle rises up while the other 

 end adheres to the liquid ; or if the level be a little too high, the 

 needle often floats by capillary action and so gets shifted off from 

 its support. Moreover the needle rusts and varies in mag- 

 netic intensity, which last circumstance was noticed by Plateau. 



afterwards published in this Journal and elsewhere, chemical cleanliness is 

 insisted on, and directions are given for obtaining it, as a necessary condi- 

 tion to success in obtaining a large number of varied phenomena.: — Tr. 



