The density of sirup is also affected by the amount and rapidity of
evaporation that takes place in boiling, and these, in turn, depend on
the amount of surface that is exposed. For instance, if a sirup is
cooked in a large, flat kettle, the evaporation will be greater and more
rapid than if it is cooked in a small, deep vessel. Atmospheric pressure
affects the rapidity of evaporation, too. In a high altitude,
evaporation takes place more slowly than at sea level, because the
boiling point is lower. Thus, in the making of sirups for canning, the
first point to be determined is whether the sirup desired should be
light, medium, or heavy, and in its preparation the points mentioned
must receive consideration.
81. For determining the density of sirup, a _sirup gauge_, or
_hydrometer_, will be found useful. This device consists of a graduated
glass tube attached to a bulb that is weighted with mercury. The
graduations, or marks, on the tube, or top part, of the hydrometer serve
to indicate the percentage of solid matter dissolved in a solution and
register from to 50 degrees. To use such a gauge, partly fill a glass
cylinder--an ordinary drinking glass will do--with the sirup and place
the hydrometer in it.
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