The "glycerine" spelling while technically incorrect has come into wide spread general and commercial use.  Glycerol is present in all animal and vegetable fats and oils most usually as a triglyceride; a product of naturally occurring chemical reactions with fatty acids.  Glycerin may be produced from fats and oils by saponification, hydrolysis, or transesterification or it may be synthesized from propylene.  Glycerin is closely associated with soap making, being obtained as a by-product when the fatty acids in fats and oils used as a soap base react with an alkali to make soap.  Usually glycerin is separated from soap because it is an important material in its own right, finding application in many industries. It is a key ingredient in transparent bar soap and remains in bar soaps made by the "cold process" simply because the soap is made without removing glycerin.