Specific Gravity

Specific Gravity  - A measure of density

Two material objects of the same size, such a a wood block and a gold block, may have the same proportions or measurements but will not have the same density, or therefore the same weight.  
But like-sized pieces of one and the same material always have the same weight.

It has long been a practice to compare the densities of substances with the density of water. 

The number which expresses the relation between the density of any substance and the density of water is called the specific gravity number of the substance.

Density is mass per unit of volume.   Or, the ratio of the amount of matter in an object to its volume.  

In the late 1800’s the understanding of scientic process was much easier to grasp because the method of writing of these was much simpler, less complicated by terms and language usage of the profession.  This is a summary of what various late 19th and early 20th  century writers told of the why and how to learn of the specific gravity of minerals.  keep reading here   or here