34 
MR. FARADAY ON THE 
found to be a practicable process; because the stirring, in the manner in 
which I have yet performed it, tends to introduce bubbles into the glass ; and 
though these are only small, still they are objectionable. Hence a mixed 
process has been adopted, which, as I have before stated, is subject to cor- 
rection from future experiments. To render the process as far as it has 
been carried sufficiently intelligible to others, I will first describe the circum- 
stances connected with stirring, and their influence upon striae; and afterwards, 
the plans adopted for the dispersion of bubbles. 
74. It is not a small degree of stirring and agitation which is sufficient to 
make a fluid of mixed materials homogeneous; especially when the mixture is 
not exceedingly fluid, but has a considerable degree of tenacity, something like 
tar or syrup. An idea of the extent to which it must be carried, and of the 
general nature of strice in fluids, may be gained by taking a glass full of clear 
saturated syrup, made from white sugar, putting a few drops of water into it, 
and stirring the whole together. It may then be remarked how slow the striae 
are in disappearing ; and when they are apparently destroyed, if the whole be 
left for some hours, it will frequently happen that a separation will take place 
into a lower heavy, and superincumbent light portion, which when stirred to- 
gether again produce striae. In the glass, the stirring must be in the utmost 
degree perfect, for if there be the least difference in different parts, it is liable 
to form striae : nor are the different portions allowed to arrange themselves 
by their specific gravities, in which case one part might perhaps be removed 
from another, after the glass was finished and cold ; but the ascending and 
descending currents which inevitably take place in the fluid matter, are cer- 
tain to arrange the irregularities in such a manner as to produce the strongest 
possible bad effect. 
75 . The instrument used for stirring has hitherto consisted of a piece of plate 
platina, which for the seven-inch glass, taken as illustrating the process, is 6^ 
inches in length and fths of an inch in breadth. It is perforated with various 
irregular holes, that, when drawn through the glass like a rake, it may effec- 
tually mix the parts. A piece of thick platina wire, about thirteen inches long, 
i" riveted to it, and the extremity of this screwed into the end of a clean iron 
rod which answers the purpose of a handle. No small or cellular apertures 
should be allowed in this stirrer; for they will frequently retain air or moisture, 
