Intelligence and Miscellaneous Articles. 405 



I shall not dwell upon the precautions which I took with respect 

 to the illumination of the meniscus (by means of a pencil of elec- 

 tric light sifted by passing through a column of water coloured with 

 a little bichromate of potass) in order to be certain of viewing the 

 upper part of it. Here a cause of error exists which is much more 

 frequent than is generally thought, especially when the black sil- 

 houette of the meniscus is projected upon a bright ground. 



The measurements were performed with a small cathetomefer 

 of a peculiar construction, on which -},— millim. could be read off, 

 and which I had made by M. Benevolo, in my private ateber, spe- 

 cially for these researches. 



In my first investigation I descended only to 6*5 millim. pres- 

 sure ; this time I have often operated with pressures below 1 mil- 

 lim. I alwavs arrived at the result that the deviation is of the 

 order of magnitude of unavoidable errors. Indeed, for initial 

 pressures of 12 millim. (in round numbers), two series composed of 



numerous fairly concordant results gave for the value of ^—, (v 



being sensibly = 2>/) the numbers 0*9986 and 1*0020 relative to 

 air; for the initial pressures comprised between 3 and 1 millim. 

 the results varied between 0-9999 and 1-0040 ; and for pressures 

 near 1 mUlmi. the extremes are 0-999 and 1-015 : this divergence 

 corresponds to an error of 15 millim. in the measurement of the 

 pressure. All these numbers are means. 



In his experiments M. Men deleef obtained a series of products^?'. 

 This appears a more favourable condition for showing how those 

 products vary. According to him, they go on decreasing with the 

 pressure, starting from a certain pressure which would be 6 decim. 

 for air. In order to prevent any delusion in this respect, it is well 

 to observe that every sensibly constant cause of error in the esti- 

 mation of the pressures, taking effect upon smaller and smaller 

 pressures, and consequently giving a relatively greater and greater 

 error, will produce the illusion of a regular augmentation or dimi- 

 nution of the products pa. This is what must result, for example, 

 from the want of absolute vacuum in the barometric chamber, even 

 if it be only on account of the effect produced by the mercury 

 vapour. 



In short, a minute examination of the possible errors has shown 

 me that, even if one could attain in the readings the precision 

 spoken of by 31. Mendeleef (thousandths of a degree, and thou- 

 sandths of a millimetre), that pressure would be illusory in the 

 presence of errors proceeding from manifold causes — such as the 

 refraction- and capillarity-errors (which, even when the precautions 

 I have indicated are taken, are never absolutely cancelled), the 

 error due to the compelled imperfection of the barometric chamber 

 (which, causing all the pressures to appear a little too low, tends to 

 produce the illusion of a negative deviation), that due to condensa- 

 tion of gases on the sides of the vessels or even on the mercurv, 

 &c. &c. 



By admitting an error of one or two hundredths of a millimetre, 



