Second set, temp. 13 (corr.). Hour, 7.45 P.M.
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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 :
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Percent- age strength.
Time.
First value
Of ^"'2.
Final value
V 4 ' 2 -
Difference.
3-86
4 hours
1 -338412
1 -338424
-0-000018
70-04
26
1 -406094 1 -406087
-0-000007
91 -29
9 1 -400990 1 -400997
+ -000007
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
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.
