What is the liquid in Mathmos lava lamps?
The company is not very forthcoming with the ingredients in their FAQ; however, they do state in regards for disposal:


  Q. How should I dispose of my lava lamp bottle when it reaches the end of its life?
  
  A. Please make a hole in the metal cap with a sharp point. Then pour
  away the liquid. The bottle may then be placed in the glass
  recycling.


However, personally, I go by a general rule that if I do not know what substance is, I do not pour it down the sink.  The authors of the HowStuffWorks webpage How to Dispose of Lava Lamps state the actual ingredients are a trade secret, but the first patent for lava lamps states the ingredients as being:


  "solidified globule of mineral oil, paraffin and a dye as well as paraffin wax or petroleum jell, preferably Ondina 17 with a light paraffin, carbontetrachoride, a dye and the paraffin wax or petroleum jelly."


as for disposal, the author of that website made contact with a company, who advised that


  They told me that I could wrap the lava lamp in newspaper and throw it in the trash. I was told that it was non-toxic. They warned me not to pour the liquid down the sink because it contained wax.


The wax could make a mess of the pipes from your sink if it were to accumulate and solidify, and there is more than enough rubbish and pollutants in the water ways as it is.  

Best to consult your local area's rules and regulations as to disposal.