I noticed recently that Caroline is very quiet and lethargic, and that her abdomen has become very swollen and taut as a drum. Today I took her to the vet who diagnosed ascites.  He calmed her with a sedative and drained the fluid - must have been literally nearly a litre of the stuff - and gave her antibiotics.  An X-ray confirmed his diagnosis that the ascites is caused by heart failure.  She is staying there tonight, to recover quietly from the treatment, and to receive more antibiotics.  I will go back after school tomorrow and hopefully bring her home.  It seems that today's treatment is to be a regular event for the time that we have left.  The best advice I found on line is that "It's best to shower your pet with love and affection, and follow up with routine vet checks if your cat has been diagnosed with abdominal fluid accumulation" (vet info.com) as the treatment is not a cure, but is "a way to give you and your pet a little more time with each other".   I walked home in darkness.