112 Messrs. V. H. Veley and J. J. Manley. 



Second set, temp. 13 (corr.). Hour, 7.45 P.M. 



Micrometer No. I. 



Direct readings 180 0' 6" 



Deviation 180 31 12 



Micrometer No. II. 

 360 0' 8" 

 330 31 



Difference 



29 28 54 



Mean value = 29 29' 1". 



29 29 8 



In the interval which elapsed between the two sets of observations the 

 thermometer was removed and its aperture covered by a quartz plate. 

 If the two mean values given above are corrected to the same standard 

 temperature, 14 2', the value of the former becomes 29 26' 27", the 

 latter 29 26' 28", or a difference of 1". 



X. Sources of Error and their Magnitude. 



I. Effect of Prism on the Acid. In order to ascertain if any chemical 

 action occurred between the quartz and the acid, the following experi- 

 ments may be cited as a selection of those conducted. Three samples 

 of 3-86, 70-04, and 91 '29 per cent, concentration respectively were 

 chosen, and the value of their refractive indices determined (i) as soon 

 as possible after introduction and (ii) after the expiry of a known 

 period of time. The values are given in the following table : 



The differences given in the fifth column are within the limit of 

 experimental error, and therefore the amount of substance which 

 might have been dissolved out would not materially alter the value 



Of fJ-. 



II. Effect of Paraffin on the Acid. Into a sample of 29'4 per cent, 

 concentration (/A D 14 " 2 = 1-374181) a quantity of the above-mentioned 

 purified paraffin in the form of fine shavings was introduced; the 

 thermometer hole was closed with a well-polished plate of quartz 

 and the whole allowed to stand at ordinary temperature. At the 

 end of 18 hours determinations gave the value /A D 142 = 1 '374237 

 and at the end of 42 hours a value /V 42 = 1 "374258, or variations 

 of +0-000056 and +0-000077 respectively. 



