314 Messrs. Haldane and Peinbrey on an Improved Method 



modified method the rate of aspiration may be more than 20 

 times as fast and the accidental errors in weighing are reduced 

 to a sixth or less. Thus for a given duration of experiment 

 the error in the modified method is less than a hundredth of 

 that by the older method. In other words, an experiment of 

 one minute's duration by the modified method is equal in 

 accuracy to a two hours' experiment by the older method. 



It is perhaps hardly necessary to refer to the great con- 

 venience of the modified method. For experiments in the 

 open air sets of weighed tubes can be carried even for long 

 distances in a box with places arranged for the tubes. TTe 

 have found that no variation in weight is caused by the 

 shaking. &c. (see p. 317). Xo precautions beyond the very 

 simple ones above mentioned are necessary in weighing : and 

 with a good short-beam balance not more than three minutes 

 are occupied in the whole of the manipulations connected with 

 weighing. Once an absorption-apparatus is filled it is always 

 ready for use. and may be used every day for nearly a year, 

 The fact that only a small quantity (usually about 6 litres) of 

 air is needed for an analysis makes it possible to use an easily 

 portable aspirator, such as that described above. 



II. Comparative Experiments with the Dry- and Wet-Bulb 



Psychrometer and the Apparatus described above. {Made 

 at the Badclife Observatory, Oxford, by IT. S. P.) 



This series of experiments was undertaken with a view to 

 test the degree of accuracy attainable by ordinary observa- 

 tions with the dry- and wet-bulb psychrometer : and also with 

 the object of obtaining independent evidence as to the 

 accuracy of the different tables used in connexion with the 

 psychrometer. 



The chemical method has been recognized by Eegnault 

 and others as the standard with which to compare all other 

 hygrometers. How important the first of these observers 

 considered the method is shown by the following extracts 

 from his Etudes sur V Hygrometrie : — " . . . j'attaehais 

 un grand interet a rendre cette methode eminemment 

 pratique et facile a employer dans toutes les experiences 

 hygrometriques." * 



" La methode chimique convient eminemment a la verifica- 

 tion des autres methodes hygrometriques et a la determination 

 des constantes numeriques que plusieurs d'entre elles exigent. 

 J'en ai constamment fait usage, a ce point de vue, dans les 

 recherches qui font l'objet de mon premier Memoire." f 



* Annates de Chimie et de Physique, xv. p. 152 (184-5). 



* Ibid, xxxvii. p. 257 (1853). 



