544 GEOLOGY OF OHIO. 
solid slag form, while the interior portion is perfectly liquid. While, 
however, the more basic silicates, that is, those containing less silica and 
more lime, becume more liquid, when melted they fuse with greater 
difficulty. Of the other elements besides silica in these slags, lime is 
the principal. A mixture of silica and lime alone is practically in- 
fusible ; alumina also forms an infusible mixture with silica; a mixture 
of these three materials, however, forms the fusible slag of the blast 
furnaces. Many experiments made on these mixtures show that the 
most fusible compounds are those in which the silica forms from 30 to 
90 parts of the whole, and the alumina forms about 4 to + part of the 
lime. If the alumina becomes very small in proportion to the lime the 
slag becomes much more infusible. Magnesia acts as lime, but when 
mixed with lime in the slag tends to increase the fusibility of the mix- 
ture over what it would be if pure lime alone were used. This is true, 
provided the percentage of magnesia be not too great; hence, in many 
cases where alumina is so deficient in a charge as to render the obtaining 
of a proper proportion impossible, the use of magnesian limestones, con- 
taining 9 to 10 per cent. of carbonate of magnesia, is sometimes found 
advantageous. The sources of the slag forming constituents in a blast 
furnace are the ores, the limestone and the fuel; the ore can be con- 
sidered asa known and nearly constant quantity in the present discussion ; 
the limestone will vary from furnace to furnace, but for all practical 
purposes may be considered as consisting of a mixture of pure carbonates 
of lime and magnesia with silicious matter (the material left untouched 
when the limestone is dissolved in acids). The composition of this is im- 
portant. A series of analyses made in the laboratory of the State Uni- ~ 
versity, in which the silicious matter was determined, and also its con- 
tent in silica, shows that on the average this silicious matter contains 
of silica and 4 alumina, or closely approximates clay in composition ; 
the addition of lime to this would but imperfectly flux it; still it is 
customary to regard it as entirely fluxed by the lime of the limestone, 
and to deduct from the total carbonates of lime and magnesia present, 
twice the amount of this silicious matter as used up by it, leaving the 
rest available. This is a sufficiently near approximation to the truth 
to serve as a means of comparing different limestones for furnace use. 
The third of the series of slag making materials is the ash of the fuel; | 
this may be regarded as having a mean composition, as follows: 
