730 



SCIENCE. 



[N. S. Vol. VII. No. 178. 



Variability is the expression of the funda- 

 mental energy of the organism, and is not 

 an Irregular accident. Heredity is the ex- 

 pression of the acquired adjustment of the 

 organism to the conditions of its existence. 

 Mutable heredity sounds like a contra- 

 diction ; so did mutable species a century 

 ago ; but it is only as heredity is mutable 

 that evolution is possible. 



THE 3IEA8UBEMENT OF S3IALL GASEOUS 

 PBESSUEES* 



Peiob to the invention of the McLeod 

 vacuum gauge, the measurement of even 

 moderately small gaseous pressures was 

 difficult, and subject to large errors. The 

 introduction of the McLeod gauge, however, 

 early in the seventies, seemed to solve the 

 problem. In its ordinary form, and for 

 most purposes, this beautiful instrument 

 admirably serves the purpose for which it 

 is designed. But when very accurate meas- 

 urements of pressures as small as a few 

 miillionths only of atmospheric pressure 

 are desired, its performance is extremely 

 unsatisfactory and vexatious. As is well 

 known, the chief cause of the difficulty is 

 the unequal and variable capillary depres- 

 sion of the two small columns of mercury, 

 whose difference in height indirectly serves 

 as the measure of pressure. Accurate 

 measurement of this capricious difference 

 obviously avails nothing. 



Three or four years ago I was engaged 

 in an investigation requiring frequent and 

 simultaneous measurements of slight but 

 different pressures in two large glass globes 

 connected by a capillary tube. For this 

 purpose I constructed and carefully cali- 

 brated two large McLeod gauges. The in- 

 ternal diameter of the mercury tubes was 

 about three millimeters, and they were 

 made from contiguous parts of the same 

 glass tube selected for uniformity of bore. 



* Bead before the American Association for the Ad- 

 vancement of Science, August 12, 1897. 



These gauges were often compared by 

 measuring the same vacuum with both, 

 but they rarely gave concordant results. 

 Indeed, it was not uncommon at high ex- 

 haustions for one or the other of them to 

 indicate a negative vacuum ; that is to say, 

 less than no pressure at all. The case of 

 these two gauges is cited because of the op- 

 portunity they afforded for comparison. In 

 prior work I had, like most experimental- 

 ists, used but one gauge, and, while always 

 suspicious of its indications, had no means 

 of knowing how large its errors might be. 



The phenomenon which I next desired to 

 investigate is the spontaneous evolution of 

 of gas from glass and other surfaces in high 

 vacua. For this purpose an accurate and 

 entirely reliable means for measuring very 

 small pressures was necessary, because I 

 could not afford to wait months or years for 

 the evolution of sufficient gas to be detected 

 with certainty by the old gauges. To meet 

 these requirements, I designed, constructed, 

 and learned how to use, the modified form 

 of McLeod gauge, which it is the purpose of 

 this paper to discuss. 



The diagram herewith shows the essen- 

 tial parts of my apparatus. The bulb A, of 

 the gauge, is made conical in its upper part 

 to avoid adhesion of gas bubbles when the 

 mercury rises. This bulb holds about 

 eleven pounds of mercury. B and C are 

 the gauge head and comparison tube re- 

 spectively. They are nearly twenty milli- 

 meters inside diameter, and are made from 

 contiguous parts of the same carefully se- 

 lected tube. D is the usual air trap, and 

 E is a long glass tube, with flexible pure 

 rubber connections to the lower end of the 

 gauge stem and the mercury cistern F. 

 The latter is mounted on a carriage G, 

 which moves vertically on fixed guides. 

 The height of the carriage is adjustable, at 

 the upper end of its range of motion, by 

 means of screw H, thumb-nut I and forked 

 support K. The screw is pivoted to the 



