By: Abdulkader M. Abed
3.6- Specific Gravity
This is another important property coined by Al-Biruni, where he measured the specific gravity of several minerals for the first time to my knowledge. He invented a simple apparatus to do this (Fig. 3). The procedure may be simple, but the results are accurate and reproducible. The conical apparatus is filled with water to the mark. Then, a piece of the mineral is weighed and put in the apparatus. The volume of the displaced water is determined, which equals the volume of the piece of the mineral. The specific gravity of the mineral was calculated in reference to a standard volume of the yellow-orange Yaghout (corundum), and not to water as we now calculate it. The specific gravity of the standard (Kutb) yellow-orange Yaghout was 100 Mithkal. In other words, he was comparing the weights of equal volumes of the minerals and the Kutb Yaghout. The experiment was the same as the current practice for measurement, except that we now use water as the reference material with a specific gravity of 1. Table 1 shows the specific gravity of some minerals and materials relative to Kutb Yaghout.
Dawanik are parts of Mithkal, Tasaseej are parts of Dawanik.
These numbers are of little use until they are converted to specific gravity on the basis of water = 1 i.e. to numbers that can be compared to what we have in modern mineralogy books. To do this, find out the modern specific gravity of Yaghout or corundum (it ranges from 4.01 to 4.4), this is equal to 100 Mithkal or 2400 Tasaseej. Then convert all other numbers to get the modern specific gravity for the above gems as shown in Table 2. Note that the modern specific gravity for the above gems is a range rather than a single value, because of impurities inherited in their genesis. Also, the exact composition of the reference variety of Yaghout used by Al-Biruni is not known. Consequently, some differences are present. However, they are small enough to indicate the accuracy of Al-Biruni’s experiments.
4- Experimentation
One should not expect Muslim scientists to have performed complicated experiments using sophisticated equipment in the manner of present day mineralogy. Experiments were focused on the physical properties of the minerals, in order to differentiate or identify them. Some of the Muslim scientists such as Ibn Masawaih had slightly experimented on minerals; others such as Al-Tiefashi had frequently used experiments; and a third class of scientists such as Al-Biruni acquired all their knowledge of minerals through experimentation.
A short example can be found written in Al-Biruni’s book:
“Al-Jamahir …” p. 58, while talking about Yaghout (ruby): “…I (AI-Biruni) bought some raw pebbles brought from India. I heated some of them, they became more red. There were two very dark pieces, one was with reddish colour, the other was less red. I put both pieces in a crucible and directed the flame at them for a period sufficient to melt 50 Mithkal of gold. I took the pieces after cooled. I noticed that the less red piece became purer with a rose red colour. The other, deep red piece lost its colour and became like Sarandeep (now Sri Lanka) quartz. I then examined this latter piece and found that it was softer than the Yaghout I concluded: when redness is lost with heating, the heated material is not Yaghout. This conclusion cannot be reversed; i.e. if the heated material stays red it is not necessarily Yaghout, because iron stays red after heating”. (Author’s translation).
There are several tests in the above paragraph:
Al-Biruni first classified the pebbles as red yaghout (ruby).
Two pieces were doubtful on the basis of their colour.
The colour was tested by heating the two pieces.
They were then tested for their hardness.
A general conclusion was made for the whole work.
There are many other experiments where more than one physical property is used to identify a mineral or a gem.
5- An Example of Other Writings
Let us end this article by giving an example of other writings in the field of mineralogy. The example is Sabaj (known now as jet). The writings on diamond or Yaghout are too long to copy.
“This is not a gem……………… It is a deep black stone, sageel, very soft, light, and burns with fire. It ignites if heated by the sun’s rays and has a petroleum smell which is in agreement with our (his) descriptions. It is a solidified petroleum very much like the black stones used to heat ovens in Farghanah (now in Uzbekistan). This is because the asphalt, bitumen, and petroleum rises in the mountains in Farghanah. few lines on the metals and minerals in Farghanah, … , The best Sabaj quality comes from Tus (now in Iran), it is used to make mirrors and utensils, and usually found in black, wetted, rotten land……………….. It is also brought from the Dead Sea area. Its specific gravity relative to Yaghout is approximately 28. The type brought from Samarkand has a specific gravity of 26.25 (around 1 relative to water). I (He) fixed this weight relative to Yaghout because of the abundance of bubbles within it which increase the volume and decrease the weight.” P. 68, “AlJamahir…” (Author’s translation)
Note the wealth of information in this paragraph:
Areas from which it is brought or mined
Detailed physical properties
Variation in specific gravity and its causes
Genesis: its relation to petroleum
Uses