104 PROCESS OF MANUFACTURE. 
with the wood; a shovelful thrown in now and then is 
very useful in quickening the fire, but care mnst be taken 
to keep the grate bars from becoming clogged with cin- 
ders. 
The part played by the chemical agents in this clarifica- 
tion has not been adverted to. It will be noticed on a 
subsequent page. The most important advantages result- 
ing from their use are not all made apparent at once to the 
eye; but the visible evidences of purity —transparency, 
brightness, and lightness of color——-are not produced to an 
equal extent by the use of any other means which I have 
been able to discover, or that I believe may be innocently 
employed. 
When these clarifying agents cannot be obtained, lime 
may be substituted for them, but it is far less efficient. A 
quantity of the cream of lime should be prepared before- 
hand (Chap. XXX.), of uniform consistence and strength. 
The exact measure of it which is necessary to be added to 
a given measure of the juice to cause it to exhibit the 
faint alkaline reaction required, should be ascertained and 
noted. Successive batches of the fresh juice may after- 
ward be tempered with the same proportion of lime pre- 
viously used when it is certain that there is no variation in 
the quality of the juice.. But .much caution is here neces- 
sary to avoid error, for the degree of acidity is not constant 
even in the juices of canes taken from the same field, and 
the means of readily detecting these differences must be 
at hand, and frequently used. The sense of taste may in- 
deed acquire an acuteness in detecting minute differences 
of quality in the juice, scarcely inferior to that of chemical 
reagents; but until it has acquired this delicacy, we must 
rely upon the test paper alone. 
