158 REPORTS ON THE STATE OP SCIENCE. 



e.g., given a reading of 2"15 per cent, at 20° and 770 mm. Hg, what is 

 the percentage ? 



2-15 per cent. = 86 milligrammes. 



log P = 1-8377 + log 86 - log 1054 + log 293 - log 770 

 1-9345 3 0228 



add 2-4669 2-8865 



6-2391 5-9093 



subtr. 5-9093 



0-3298 

 P = 21 37, instead of 2150. 



The correction is — 0-013. 



Take an extreme case of a balance used at St. Moritz at tempera- 

 ture =20° and pressure = 650 mm. Hg. Reading as before = 2'15 



log P = 1-8377 + log 86 - log 1054 2 — or + log 293 - log 650 

 1-9345 3 0228 



2-4669 2-8129 



6-2391 5-8357 



5-8357 



0-4034 

 P = 2-531, instead of 2-15. 



The correction is +0-381. 



Adjustment of the Scale to the Index. — With a chloroform-balance 

 of type A there is, as stated above, no need to take account of variations 

 of temperature and pressure during administration. The position of 

 the index in front of the scale (or if the instrument is used as a recorder 

 the position of the pen against the recording surface), with only air in 

 the balance-case is the zero or point of departure for measurement (or 

 record) of the percentage of chloroform vapour subsequently present. 

 The position of that zero will be found to vary slightly on different days 

 by reason of differences of temperature and pressure; as regards the 

 record, if such be taken, this variation of position of the zero line is 

 of no account, but if we are taking for our guide the position of the indi- 

 cator in front of a fixed scale it is inconvenient to have a zero that does 

 not coincide with the zero marked on the scale. The discrepancy, if 

 considerable, can be corrected by alteration of the counterpoise, but it 

 is more convenient to slide the scale slightly to the right or left by 

 means of a screw adjustment until its zero coincides with the zero 

 position of the index. 



Readings of the scale are best taken with the beam of the balance 

 and the indicator oscillating freely right and left of a middle point, which 

 is the actual reading. The oscillations can be clamped so that readings 

 are taken with the indicator at rest by means of a light bristle fixed to 

 the indicator and rubbing lightly against the glass surface of the scale. 

 But if this device is employed it must be verified as not liable to cause 

 the indicator to stick in consequence of excessive friction so as to give 

 false readings. 



When a record of the administration is taken — a proceeding which 

 I do not recommend for the ordinary use of the apparatus — a satisfac- 

 tory damping of oscillations is afforded by the point of contact of the 



