In this tier is also found a good deal of mineral wool, which must not be mistaken for asbestos.
One of the best and most available materials at present known for this purpose is 'mineral wool,' a product of iron 'slag.'
Asbestos and mineral wool are the best, and have the additional advantage that they cannot burn.
This may be filled in with concrete, mineral wool, or other non-conducting material.
All the walls and partitions around it will be double and filled with mineral wool, and the floors will be deafened.
This does not act as deadening, unless concrete or mineral wool be placed over it.