It seems it was a series of letter to Father Campi: " In the last of the letters he wrote to Father Campi on the matter he suggested it [methane] might be used in lamps instead of oil. He himself was the first scientist to build a perpetual lamp or Volta lamp. He applied a discharge from his new electrical invention, the electrophorus, generating a spark that to light it, or to exploded the marsh gases. "
Deepak. Please come here to Lake Como and River Adda and we'll see if we find the exact place where Alessandro Volta was observing methane bubbling out from sediments. :-)))